The Bible.
As I said before, eternal life is not a super power or some kind of wish that is granted to you by some genie. Eternal life is a property that is exclusive to the Son of God. For he that has the Son has life and he that does not have the Son does not have life (1 John 5:12). Jesus alone possesses immortality (1 Timothy 6:16). This lets us know that you cannot be out of fellowship with God and still be saved (Which goes against the popular belief held by many OSAS proponents).
Actually, here is a complete list of passages (I found) that tell us that you cannot be out of fellowship with God and be saved.
#1. 1 John 5:12 says He that has the Son has life and He that does not have the Son does not have life. Life is associated with eternal life or salvation.
#2. John 17:3 says eternal life is in knowing the one true God, Jesus Christ. Knowing implies a fellowship. So if you don't know Jesus, then you don't have life (Salvation).
#3. Romans 8:9 says if he a man does not have the Spirit of Christ, he does not belong to him.
#4. Psalm 73:27 says God will destroy all those who abandon Him (or go a whoring from Him).
#5. John 15:6 says if a man does not abide in Him, he is cast forth and burned.
#6. 1 John 1:7 says if we walk in the Light as He is in the Light, the blood of Jesus Christ cleanses us from all sin.
#7. Romans 11:21-22 says if you do not continue in his goodness you will be cut off. For if God spared not the natural branches (i.e. the Jews), take heed that he can do the same to you (i.e. Gentile believers). The analogy here is that you are branch and Christ is the tree. We need to continue in Christ's righteousness or goodness, not our own righteousness or goodness, or we will be cut off because of unbelief.