Can a saved person ever become unsaved, or lose their salvation?

The book of 1 John states: "These things I have written to you who believe in the Son of God, in order that you may know that you have eternal life.(1 John 5:13)." Notice that the Apostle says you now have eternal life. We don't get eternal life after we die... we have it right now, when we accept Jesus Christ as our personal savior.
Now one might ask himself, how long does eternal life last? Can you have eternal life for a year? A week? A day? If words mean anything at all, then eternal life must be eternal. It must last forever! Eternal life by definition cannot be lost.
Now what sin is so big that Jesus' death could not atone for it? Is there one that you know of? In Romans 8:38-39 explicitly states this, saying "For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, shall be able to separate us from the love of God, which is in Christ Jesus our Lord." I am a created thing. The verse above states that created things cannot separate us from the love of God, therefore I cannot separate myself from the love of God, either!
Also, in John 10:29, Jesus tells us "And I give eternal life to them; and they shall never perish, and no one shall snatch them out of My hand. My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of the Father's hand."
In Heb 6:4-6, it reads: "For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
This on the other hand means that if it were possible to lose your salvation, Jesus would have to come back to earth and die again and be shamed for having to do this. When Jesus died on the cross for our sins he raised his head to the heavens and said "It is Finished”, Not "It is done for now". When Jesus stated "It is finished" he meant just that. That would be the final and greatest sacrifice for our sins.
Which means there is no need for an alter ever again, but that is another subject so I'll leave it at just that.

The Old Testament

The New Testament