This is a very interesting question.  When we put our faith in the Lord Jesus Christ, the Lord gives us eternal life.  The very definition of eternal life is a life that NEVER ENDS.  I will list a few verses that show that once we are saved, or become a Christian, we will never lose that salvation.  Let’s let the Word of God speak for itself:

• John 10:27-28, “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.”
• John 5:24, “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.”
• Romans 8:38-39, “For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.”
• Hebrews 7:25, “Wherefore he is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them.” 

We could go on and on, showing the ETERNAL LIFE that is given to all those who believe on the Lord Jesus Christ as their savior.  As we saw in Romans 8:38-39 there is nothing that is, “able to separate us from the love of God, which is in Christ Jesus our Lord.”

But…is it possible for someone to give up their salvation?  Can someone who truly put his faith in the Lord Jesus stop believing in Him?  Can someone denounce the Lord after he has been saved by His amazing grace? 

Again, let’s let the Word of God speak for itself.  We read in Ephesians 1:13-14, “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 of our inheritance until the redemption of the purchased possession, unto the praise of his glory.”  When one accepts the Lord Jesus as his savior, he is immediately indwelt and sealed with the Holy Spirit.  (Read Ephesians 4:30).  So, since we are sealed with the Spirit, we understand He will stay with us and in us until the day of the redemption of our bodies.  One who is truly saved and indwelt with the Spirit could never denounce the Lord as his savior.  We read in Romans 11:29, “For the gifts and the calling of God [are] not subject to repentance.” (DARBY).  This means God does not change His mind towards those whom He has called and saved.  Our salvation is irrevocable.  It is irreversible.  It is permanent. 

The Apostle John was led to address this issue in his first epistle.  He said, “They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us.” (1 John 2:19).  If one professes to be saved, but later leaves that profession and says he no longer believes in Christ, he was never saved in the first place.  As we just read, “THEY WERE NOT ALL OF US.”  I’m sure most of us have witnessed someone who seemed to be following Christ for a while but later reverted back to his old ways and stopped having a testimony of salvation.  I believe this verse applies to that type of person. 

You asked, “Is it possible for an ex-Christian to become a Christian again?”  To that I would respond by saying, “It is impossible for a true Christian to become an ex-Christian.”  (CC)  (674.4)