Could you give me the Scripture I need to show someone that we are once saved, always saved?

Christ tells us in John 6:37 there is no way God is going to take away our eternal life. All that the Father giveth me shall come to me: and him that cometh to me I will in no wise cast out".

Some will say, "All right. God won't cast me out, but I could get out of my own will and choice!" Go to John 6:39, "And this is the Father’s will which hath sent me, that of all which he hath given me I should LOSE NOTHING, but should raise it up again at the last day." This verse tells us it is God's will that NO saved person would ever become lost. He says He will "lose nothing", and you are at least something! Actually, we are so important to God that He sent His only Son from Heaven to earth to die in payment for our sin!

I Peter 1:4,5 lets us know that it is God who keeps us. God, himself, by His mighty power, is reserving a place in Heaven for us. "Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively (living) hope by the resurrection of Jesus Christ from the dead". "To an inheritance incorruptible, and undefiled, and that fadeth not way, RESERVED in heaven for you".

If a saved person could go to Hell, the Holy Spirit would have to go to Hell, too. He indwells every believer from the moment he trusts Christ forevermore. 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." "Which is the EARNEST (down payment) of our inheritance until the redemption of the PURCHASED possession, unto the praise of his glory." (Ephesians 1:13,14). (Also see I Corinthians 6:19; and John 14:16,17).

John 10:28 "nails it down"! Christ gives eternal life, and they who receive this eternal life "shall never perish". "Never" in the original Greek is a double negative, meaning, "not at all, by no means, in no case, NEVER"! "And I give unto them eternal life; and they shall NEVER perish, neither shall ANY MAN (not even yourself) pluck them out of my hand."