Eternal security is the doctrine that when a person is saved nothing can affect their salvation. They can never lose it for any reason. I believe Eternal Security is true because of the following verses:
John 3:16
For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.
Well, believing in Jesus is more than just having a mental picture of Jesus in your mind that He is your Savior alone. Remember, when Jesus said to Peter after he failed to walk on the water? He said to Peter, "Oh, you have little faith." So a lack of proper action in one's life is a lack of faith.
We are told in Hebrews 3,
"Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin." (Hebrews 3:12-13).
So you can have an evil heart of unbelief and depart from the living God. You can be hardened by the deceitfulness of sin.
But in Eternal Security, you do not need to worry about hardening yourself to the deceitfulness of sin. You got your golden ticket to Heaven. For in the OSAS belief: No believer can have a heart of unbelief and depart from the living God (even though Hebrews 3 warns us believers about this very thing).
In addition, believing in Jesus also involves believing in everything He taught and commanded us, too. If we are not obeying Jesus, then we really do not believe His words. For example: If you believed in a Jesus that told you to jump off a bridge and you were taking those words seriously, then I would say you are believing in another Jesus. You think Jesus taught that we can disobey Him on some level and yet we can still be okay with Him salvation wise. But that would be the exact opposite of what He had taught.
Jesus says in Matthew 5:22, "...whosoever shall say, You fool, shall be in danger of hell fire." In Matthew 5:28-29, Jesus says, "...whosoever looks on a woman to lust after her has committed adultery with her already in his heart. And if your right eye causes you to offend, pluck it out, and cast it from you: for it is better for you that one of your members should perish, and not that your whole body should be cast into hell." In Matthew 6:15, Jesus says, "...if you forgive not men their trespasses, neither will your Father forgive your trespasses."
If you think these words of Jesus are now undone because the New Covenant began with Christ's death, then think again. Even John says in his epistle that if you hate your brother, you are like a murderer and no murderer has eternal life abiding in him (See 1 John 3:15). In fact, John says in 1 John 3:10, "In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother."
James says even the demons believe and tremble and he says that faith without works is dead (James 2:17) (James 2:19). There is the "Law of "Faith" (Romans 3:27), and the "work of Faith" (1 Thessalonians 1:3) (2 Thessalonians 1:11) (Which is not the same thing as the Law of Moses (Luke 2:22) or the Oldness of the Letter (Romans 7:6) or the "works of the Law" (Galatians 3:2)). James says we are justified by works and not by faith alone (James 2:24). These works would obviously be a part of our faith in Christ and His teachings. For Paul says if any man speaks contary to the words of Jesus Christ and the doctrine of Godliness, he is proud and knows nothing (1 Timothy 6:3-4). James says God resists the proud and gives grace to the humble (James 4:6).
True faith always led to right action in Hebrews 11.
If you were to keep reading in John 3, you also would read this passage.
19 "And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil.
20 For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved.
21 But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God." (John 3:19-21).
Did you catch that? Verse 20 says everyone who does evil hates the light (Christ) and neither comes to the light (Christ) unless their deeds are reproved (i.e. they repent).
You said:
John 6:37-6:40
"All those the Father gives me will come to me, and whoever comes to me I will never drive away. For I have come down from heaven not to do my will but to do the will of him who sent me. And this is the will of him who sent me, that I shall lose none of all those he has given me, but raise them up at the last day. For my Father's will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise them up at the last day."
The passage says "looks" to the Son. "Looks" is an action; And what exactly are you looking at when you look at the Son? Are you only looking at the Son as described by OSAS or you looking at Jesus as described by the Bible? Jesus is described as saying things like, "If you love me, keep my commandments." (John 14:15). Do you truly love Jesus in this way? Jesus says, "you are my friends, if you do what I command you (John 15:14). Jesus says if you will enter into life, keep the commandments (Matthew 19:17). This is the version of the Son descibed in the Bible that you are to look upon. In fact, we are told to imitate Christ in Scripture (1 Peter 2:21). This is the true way of looking upon Christ. Also, John 6 uses the word "should" in verse 39. That He "should" lose nothing. The word "should" is not a guarantee. For example: If I said to you, "I should be over your house tomorrow, but I have a lot of work to do." You would not take that as a guarantee.
You said:
John 10:27-29
"My sheep listen to my voice; I know them, and they follow me. I give them eternal life, and they shall never perish; no one will snatch them out of my hand. My Father, who has given them to me, is greater than all; no one can snatch them out of my Father's hand."
You are not reading and believing vese 27. Verse 27 says that the type of sheep that will not be snatched out of the Lord's hand are the type of sheep who FOLLOW Him. For his sheep listens to His voice and He knows them and they FOLLOW Him.
You said:
Tell me, how can you read those verses and NOT believe in Eternal Security? Jesus is extremely clear. Out of all that the Father gives to Jesus he will lose NONE of them! In other words, Everyone's salvation is Eternally secure! It's right there in black and white! So, what do you think? Is Eternal Security real? Why or why not?
Eternal Security or Once Saved Always Saved (OSAS) is wrong because it teaches that you can turn God's grace into a license to sin on some level. Jude 1:4 (NIV) says, "For certain individuals whose condemnation was written about long ago have secretly slipped in among you. They are ungodly people, who pervert the grace of our God into a license for immorality and deny Jesus Christ our only Sovereign and Lord."
God is good and He cannot agree with any person's idea that they can do evil with the thinking they are saved because that would mean God would have to agree with sin (And that is just not possible). For God is good, holy, righteous, and loving. Eternal Security is morally wrong and cannot even found in Scripture (unless one ignores a ton of Scripture).