"My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand. "
John 10:27-29
"Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead, To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you, Who are kept by the power of God through faith unto salvation ready to be revealed in the last time." 1 Peter 1:3-5
"Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life." John 5:24
1) "hath" is a present condition. You receive everlasting life the moment you believed in the gospel
2) "kept by the power of God" means man does not have power to snatch himself away. He keeps an inheritance for you in heaven that does not corrupt!
3)"neither shall ANY man pluck them out of my hand". ANY man means ANY. You who believed are part of "any man". Your friend is part of "any". No man(not even you) can pluck yourself out of God's hand. And we already established that works after salvation are a fruit and have no saving power.
So to raise the "what if" question that someone just stops believing in the gospel after He believed does not make sense(or you are asking the wrong question)......He has the Holy spirit in us sealing us for the day of redemption(Ephesians 4:30) when we believe..."In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise," Ephesians 1:13
"Being confident of this very thing, that he which hath begun a good work in you will perform it until the day of Jesus Christ" Philippians 1:6