No in this you are not correct. One sin does not separate a Christian from God. God gives every person, the believer included TIME to repent from their sins. The fact is that sin has a hardening effect.
Heb 3:12-13 Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.
This hardening effect effects a person in the following way. If a person sins, they begin to have doubts, around their security, it gives the devil access to condemn. The person will feel terrible, they will have guilt. If this guilt is not addressed though eventual repentance, the person will begin to feel un-forgiven, and may walk themselves away from faith, never to return. But if at any time the person repents (and turns from the sin) God will forgive them.
Job 36:9-12 then he declares to them their work and their transgressions, that they are behaving arrogantly. He opens their ears to instruction and commands that they return from iniquity. If they listen and serve him, they complete their days in prosperity, and their years in pleasantness. But if they do not listen, they perish by the sword and die without knowledge.
Notice that if the person does not repent, they will die "without knowledge", this knowledge if the knowledge of salvation, or faith in God.
2Th 2:10-12 and with all wicked deception for those who are perishing, because they refused to love the truth and so be saved. Therefore God sends them a strong delusion, so that they may believe what is false, in order that all may be condemned who did not believe the truth but had pleasure in unrighteousness.
God allows certain people to be deluded, to no longer believe the truth. But notice He uses the word "BELIEVE what is false".
Every Christian is saved by faith, in the finished work of the cross, but if they persist in sin Jesus "spews them out of his mouth".
So we must live soberly, even though we are saved by faith.
Your doctrine is a wrong one, one sin does not kill a believer and send them to hell. Persistent sin, leading to death, does.
No. Hebrews 3:12-13 is talking about having a hard heart (via by sin) whereby they would depart from the living God. Meaning, they would apostatize or reject Christ in some way.
Anyways, the Bible clearly teaches that it only takes one sin to die spiritually.
[God said to Adam,]
"But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die." (Genesis 2:17).
[Eve said to the serpent,]
"But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die. (Genesis 3:3).
And the serpent said unto the woman,
"Ye shall not surely die." (Genesis 3:4).
"...she took of the fruit thereof, and did eat, and gave also unto her husband with her; and he did eat. And the eyes of them both were opened, and they knew that they were naked..." (Genesis 3:6-7).
"Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned." (Romans 5:12).
28 “But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
29 And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
30 And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.” (Matthew 5:28-30).
“But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul.” (Proverbs 6:32).
“But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.” (Matthew 6:15).
"For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins." (Hebrews 10:26).
"Repent therefore of this thy wickedness, and pray God, if perhaps the thought of thine heart may be forgiven thee." (Acts of the Apostles 8:22).
"He that saith he is in the light, and hateth his brother, is in darkness even until now." (1 John 2:9).
All liars will have their part in the Lake of Fire. Not just some. If one lies just once, they are a liar (unless they seek forgiveness of such a sin with Jesus).
"But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, shall have their part in the lake which burneth with fire and brimstone: which is the second death." (Revelation 21:8).
No murderer has eternal life abiding in him. Meaning any murderer. For how many murders does it take for a person to be a murderer?
"Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him." (1 John 3:15).
Then there is Ananias and Sapphira. They lied to the Holy Ghost and were instantly killed. They each lied (Which is a one time sin for each of them). When the church heard about this, a great fear came upon them. Now, if they were never saved to begin with, the emotion of fear would not make any sense because why be afraid for those who are unbelievers who deserve what they get for living sinful and unbelieving lives? If Ananias and Sapphira were saved believers: It still does not make sense that the church would experience the emotion of fear. Why would they fear if they are safely in the arms of Jesus? Is not to live is Christ and to die is gain? The only reason why the emotion of fear makes sense for the church over their deaths is if the something similar could happen to them, too.