Mark 12:26-27 And as for the dead being raised, have you not read in the book of Moses, in the passage about the bush, how God said to him, ‘I am the God of Abraham, and the God of Isaac, and the God of Jacob’? He is not God of the dead, but of the living; you are quite wrong.” RSVCE
The above of course was written in regards to the resurrection of the dead, not the dead as though they are alive. Which is why no doubt you were so selective in that quoted.
Mrk 12:18 Then come unto him the Sadducees, which say there is no resurrection; and they asked him, saying, 19Master, Moses wrote unto us, If a man's brother die, and leave his wife behind him, and leave no children, that his brother should take his wife, and raise up seed unto his brother. 20 Now there were seven brethren: and the first took a wife, and dying left no seed. 21 And the second took her, and died, neither left he any seed: and the third likewise. 22 And the seven had her, and left no seed: last of all the woman died also. 23 In the resurrection therefore, when they shall rise, whose wife shall she be of them? for the seven had her to wife. 24 And Jesus answering said unto them, Do ye not therefore err, because ye know not the scriptures, neither the power of God? 25 For when they shall rise from the dead, they neither marry, nor are given in marriage; but are as the angels which are in heaven. 26 And as touching the dead, that they rise: have ye not read in the book of Moses, how in the bush God spake unto him, saying, I am the God of Abraham, and the God of Isaac, and the God of Jacob? 27 He is not the God of the dead, but the God of the living: ye therefore do greatly err.
Hebrews 11 speaks of Abraham and Moses and others, and then Hebrews 12 says:
Hebrews 12:1 Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with perseverance the race that is set before us,[
a RSVCE
This great cloud of witnesses is made up of saints.
The witnesses are of course the deeds of the saints before us, which had just been recited in the previous chapter. Their lives as they lived and were recorded in holy scripture is the witness being spoken of. Not lives in heaven now, which could not be seen now, even if they were being lived. The testimony or witness of the first person recited in Hebrews 11, describes the type of testimony and witness of the rest in that chapter also.
Heb 11:4 By faith Abel offered unto God a more excellent sacrifice than Cain, by which he obtained witness that he was righteous, God testifying of his gifts: and by it he being dead yet speaketh.
Revelation 6:9-10 When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne; they cried out with a loud voice, “O Sovereign Lord, holy and true, how long before thou wilt judge and avenge our blood on those who dwell upon the earth?” RSVCE
So, you think all the souls that died for the word of God have been and are now under some alter in heaven waiting around for God to avenge their blood?
Gen 4:8 And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him. 9 And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother's keeper? 10 And he said, What hast thou done? the voice of thy brother's blood crieth unto me from the ground.
Do you therefore also believe that the blood of Abel of the above verses literally cried out to God? Rather than the obvious symbolism being employed? Why would you make those out to be a living souls trapped under an alter in heaven when holy scripture tells us so many times over that souls have, do, and will die? Nevertheless, so be it unto you, as you wish.
Ps 6:2 Have mercy upon me, O LORD; for I am weak: O LORD, heal me; for my bones are vexed. 3 My soul is also sore vexed: but thou, O LORD, how long? 4 Return, O LORD, deliver my soul: oh save me for thy mercies' sake. 5 For in death there is no remembrance of thee: in the grave who shall give thee thanks?
In the above, David seeks to have himself, that is his soul delivered from death, because in the grave there is no remembrance of God, and he cannot give God thanks from the grave. How is this possible if the soul is immortal? If the soul were alive and in heaven with God surely it would be praising Him. Or if it were in hell, surely it would be cursing Him.
Ps 7:1 O LORD my God, in thee do I put my trust: save me from all them that persecute me, and deliver me: 2
Lest he tear my soul like a lion, rending it in pieces, while there is none to deliver. 3 O LORD my God, if I have done this; if there be iniquity in my hands; 4 If I have rewarded evil unto him that was at peace with me; (yea, I have delivered him that without cause is mine enemy

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Let the enemy persecute my soul, and take it; yea, let him tread down my life upon the earth, and lay mine honour in the dust. Selah.
What sense do the above verses make if a soul is not a living person, rather than some floating entity that lives apart from the body? Can a soul be torn to pieces? Will it end when ones life does? Yes it will.
Ps 30:2 O LORD my God, I cried unto thee, and thou hast healed me. 3 O LORD, thou hast brought up my soul from the grave: thou hast kept me alive, that I should not go down to the pit.
How can a soul go to the grave? If it is immortal and goes to heaven or hell at death it should not be spoken of as dying and going to the grave. If on the other hand, it is a living being, then it could be said that ones soul goes to the grave when they die, it is its end. When life ends, it ends.
Ps 33:18 Behold, the eye of the LORD is upon them that fear him, upon them that hope in his mercy; 19 To deliver their soul from death, and to keep them alive in famine. 20 Our soul waiteth for the LORD: he is our help and our shield. 21 For our heart shall rejoice in him, because we have trusted in his holy name. 22 Let thy mercy, O LORD, be upon us, according as we hope in thee.
If our souls must be delivered from death, then they are not immortal. They are like us, they are us, when we are alive. When we are raised from the dead and given everlasting life, we will again be living souls.
Ps 40:13 Be pleased, O LORD, to deliver me: O LORD, make haste to help me. 14 Let them be ashamed and confounded together that seek after my soul to destroy it; let them be driven backward and put to shame that wish me evil.
Who can destroy a soul if it is immortal? It is not. When life ends, it ends, because when one is alive, they are a living soul.
Ps 49:12 Nevertheless man being in honour abideth not: he is like the beasts that perish. 13 This their way is their folly: yet their posterity approve their sayings. Selah. 14 Like sheep they are laid in the grave; death shall feed on them; and the upright shall have dominion over them in the morning; and their beauty shall consume in the grave from their dwelling. 15 But God will redeem my soul from the power of the grave: for he shall receive me. Selah.
The soul being associated with life, being redeemed from the grave. If it were immortal, this would not be so.
Ps 56:12 Thy vows are upon me, O God: I will render praises unto thee. 13 For thou hast delivered my soul from death: wilt not thou deliver my feet from falling, that I may walk before God in the light of the living?
When God delivers the soul from death, one can walk in the light of the living. Makes no sense if the soul is immortal.
Ps 78:49 He cast upon them the fierceness of his anger, wrath, and indignation, and trouble, by sending evil angels among them. 50 He made a way to his anger; he spared not their soul from death, but gave their life over to the pestilence; 51 And smote all the firstborn in Egypt; the chief of their strength in the tabernacles of Ham:
God spared not the Egyptians souls from death, but killed them by the plagues. Their souls died, that is, they died. Obviously their souls were not immortal.
Ps 86:1 Bow down thine ear, O LORD, hear me: for I am poor and needy. 2 Preserve my soul; for I am holy: O thou my God, save thy servant that trusteth in thee. 3 Be merciful unto me, O Lord: for I cry unto thee daily.
Who needs their soul to be preserved if it is immortal?
Ps 89:47 Remember how short my time is: wherefore hast thou made all men in vain? 48 What man is he that liveth, and shall not see death? shall he deliver his soul from the hand of the grave? Selah.
Why would a soul go to the grave at death if it is immortal and goes to heaven or hell at death? It is because the body goes into the grave at death, it is a dead soul, it is not living, because the body and the breath or spirit from God have separated.
Ps 116:7 Return unto thy rest, O my soul; for the LORD hath dealt bountifully with thee. 8 For thou hast delivered my soul from death, mine eyes from tears, and my feet from falling. 9 I will walk before the LORD in the land of the living.
Because his soul was delivered from death, he will walk before the Lord in the land of the living, he is a living soul.
Ps 119:174 I have longed for thy salvation, O LORD; and thy law is my delight. 175 Let my soul live, and it shall praise thee; and let thy judgments help me. 176 I have gone astray like a lost sheep; seek thy servant; for I do not forget thy commandments.
Makes no sense if the soul is immortal.
Isa 38:16 O Lord, by these things men live, and in all these things is the life of my spirit: so wilt thou recover me, and make me to live. 17 Behold, for peace I had great bitterness: but thou hast in love to my soul delivered it from the pit of corruption: for thou hast cast all my sins behind thy back. 18 For the grave cannot praise thee, death can not celebrate thee: they that go down into the pit cannot hope for thy truth. 19 The living, the living, he shall praise thee, as I do this day: the father to the children shall make known thy truth.
Because he was delivered from death, his soul was delivered from the pit of corruption, that is, the grave. The living, they are the ones who praise God. The dead cannot, because they are not living souls.
Ezek 18:4 Behold, all souls are mine; as the soul of the father, so also the soul of the son is mine: he soul that sinneth, it shall die.
Is God a liar? If a soul is immortal it cannot die. God says the soul that sinneth, it shall die.
Ezek 18:20 The soul that sinneth, it shall die. The son shall not bear the iniquity of the father, neither shall the father bear the iniquity of the son: the righteousness of the righteous shall be upon him, and the wickedness of the wicked shall be upon him. 21 But if the wicked will turn from all his sins that he hath committed, and keep all my statutes, and do that which is lawful and right, he shall surely live, he shall not die. 22 All his transgressions that he hath committed, they shall not be mentioned unto him: in his righteousness that he hath done he shall live. 23 Have I any pleasure at all that the wicked should die? saith the Lord GOD: and not that he should return from his ways, and live? 24 But when the righteous turneth away from his righteousness, and committeth iniquity, and doeth according to all the abominations that the wicked man doeth, shall he live? All his righteousness that he hath done shall not be mentioned: in his trespass that he hath trespassed, and in his sin that he hath sinned, in them shall he die. 25 Yet ye say, The way of the Lord is not equal. Hear now, O house of Israel; Is not my way equal? are not your ways unequal? 26 When a righteous man turneth away from his righteousness, and committeth iniquity, and dieth in them; for his iniquity that he hath done shall he die. 27 Again, when the wicked man turneth away from his wickedness that he hath committed, and doeth that which is lawful and right, he shall save his soul alive.
The soul is identified with the person. If the person lives, the soul lives. If the person dies, the soul dies. A soul is a living person with a body combined with the breath, or spirit from God.
Matt 10: 28 And fear not them which kill the body, but are not able to kill the soul: but rather fear him which is able to destroy both soul and body in hell.
God will destroy both body and soul in hell. The soul is not immortal.
Matt 16: 25 For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it. 26 For what is a man profited, if he shall gain the whole world, and lose his own soul? or what shall a man give in exchange for his soul?
How can one lose their soul, if their soul is immortal? Where could it go to get away from them.
Acts 2: 27 Because thou wilt not leave my soul in hell, neither wilt thou suffer thine Holy One to see corruption. 28 Thou hast made known to me the ways of life; thou shalt make me full of joy with thy countenance. 29 Men and brethren, let me freely speak unto you of the patriarch David, that he is both dead and buried, and his sepulchre is with us unto this day. 30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne; 31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption.
Christ was raised before His body saw corruption. Thus His soul was not left in hell, or the grave. He once again walked in the light of the living, that is, He became a living soul. This is our salvation. Our souls will not be left in the prison house of death, but we will once again be raised and receive the breath, or spirit of God in our spiritual bodies, and then we shall ever be with the Lord.
Acts 3: 22 For Moses truly said unto the fathers, A prophet shall the Lord your God raise up unto you of your brethren, like unto me; him shall ye hear in all things whatsoever he shall say unto you. 23 And it shall come to pass, that every soul, which will not hear that prophet, shall be destroyed from among the people.
Every soul that will not hear and accept Christ, will be destroyed.
Heb 10: 38 Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him. 39 But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
The soul needs saving, it is not immortal.
James 5:19 Brethren, if any of you do err from the truth, and one convert him; 20 Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins.
The soul needs to be saved from death, it is not immortal.
Rev 16: 3 And the second angel poured out his vial upon the sea; and it became as the blood of a dead man: and every living soul died in the sea.
Souls can and do die.