Regarding the question of once-saved, always-saved, the Bible teaches that salvation is based exclusively upon the merit of Jesus Christ. (094) People who don't know their Bibles will often debate this, but it is only because of their ignorance that they do so. People who study the Word of God deeply know that it presents salvation by grace through faith in God's promise of life through Jesus alone.
There are two questions regarding salvation by grace through faith. The first question is: Where does such saving faith come from? The second question is: How much impact does saving faith have upon the behavior of the person who has believed?
Saving faith comes from the confrontation of the person with undeniable evidence. (094) It is possible for a person to deny the undeniable, but it always results in a departure from reality. If prolonged long enough, the person will go crazy--because crazy people are those who refuse reality. The point we made was that faith is a response to evidence, while unbelief is a refusal of evidence. Thus, faith is not a choice of the will of man--it is a response to evidence that the man finds undeniable. However, unbelief is a choice of the will. Men cannot make themselves believe, but they can make themselves refuse to believe.
Now we want to look at the second question: How much impact does saving faith have upon the person exercising it? To answer that question, we must look to the Scriptures for illustrations. Abraham was a believer--so much so that he was called the Father of all who believe (Romans 4:11). Yet Abraham lied about the identity of his wife to save his own skin (Genesis 12:13); he slept with a woman who was not his wife and begat a son whom God rejected (Genesis 16); and lived in such a way that his faith was occasionally denied by his behavior.
David was a believer. But one of the most well-known stories about David was that he slept with another man's wife and then had the man killed so he wouldn't find out about it. Thus, David's faith did not keep him from adultery and murder.
Solomon was a believer. But he married 700 wives and had 300 concubines in direct contradiction to God's instructions for him. He also allowed apostasy to find root in the nation of Israel. His faith did not make him an exemplary man.
Jacob was a believer, but he bilked his brother out of his inheritance by deceit. He also practiced polygamy and played favorites with his children. He regularly got himself in trouble by manipulating people to get his way. His faith did not keep him from sinning.
Peter was an outstanding apostle by faith. But he also denied the Lord with an oath and later denied the faith because he was afraid of what people would think of him. He was a believer, but his faith did not keep him from doing significant evil.
My point? Believers sin and their faith does not keep them from doing so.