Apostasy: Can a person, "once saved" ever be lost?
The teaching that a child of God cannot sin as to ever be lost eternally is based upon such scripture as I John 3:9. There it says, "Whosoever is born of God doth not commit sin; for his seed remaineth in him: and he cannot sin, because he is born of God". There are other scriptures used but at this time I will focus upon just this one.
Concerning the phrase, a child of God cannot sin, it is said that this teaches that it is impossible for a child of God to sin. However, a closer look at John's writings will reveal that this is not the case at all. Writing to Christians, John says in I John 1:8 "If we say that we have no sin, we deceive ourselves, and the truth is not in us". So yes, Christians can and do sin. And because we do sin, even though we are Christians, we find in I John 1:9 that John writes "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness". Also in I John 2:1, he says "My Little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous". The summation is this: if a Christian cannot sin, then there is no point of John telling them to "sin not". And there would be no need for Jesus Christ the righteous, to be our advocate with the Father. Remember, sin separates one from God, Isaiah 59:2. Being separated from God, is being in a lost condition.
Now let's look further at the word "cannot" in I John 3:9. Someone asks you to be involved in some sinful practice, you say, "I cannot do that." This doesn't mean that it is impossible for you to be involved, but you "cannot" do such and remain a person of integrity. In Gen. 44 when Joseph's brothers were in Egypt to buy corn, Joseph inquired about their father and discovered that they had a younger brother. Joseph demanded that they bring him back to Egypt with them. Judah, one of the brothers said, "The lad "cannot" leave his father, for if he should leave his father, his father would die." Judah was not saying it was impossible for Benjamin to come to Egypt, but Judah was stating Benjamin "cannot" leave his father if his father is to continue to live. Many other examples of the word "cannot" being used in the manner described above can be found in the scriptures.
In closing, the scriptures do teach that once a man is saved, he can, by his own choosing, go back to a life of sin and be lost eternally. Peter writing concerning those who had been converted says this in II Peter 2:20-21, "For if after they have escaped the pollutions of the world through the knowledge of the Lord and Saviour Jesus Christ, they are again entangled therein, and overcome, the latter end is worse with them than the beginning. For it had been better for them not to have known the way of righteousness, than, after they have known it, to turn from the holy commandment delivered unto them".