We are to take heed that we do not consider departing from God by not believing as we should. Even though there were many miracles seen by the people which came out of Egypt still some of them could not enter the promised land because of unbelief(Hebrews 3:12-19).  Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end; While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation. For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses. But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness? And to whom sware he that they should not enter into his rest, but to them that believed not? So we see that they could not enter in because of unbelief.

Paul was always in remembrance of the grave mistakes he had made when it came to believing and serving the church of Christ. It was this reminder that allowed him to become strong in the Lord as he told his testimony when the Lord met him on the road to Damascus that changed his life forever! Paul came to realize it was out of unbelief he lived away from the one who would eventually save him (1 Timothy 1:9-15).  Knowing this, that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for manslayers, For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine; According to the glorious gospel of the blessed God, which was committed to my trust. And I thank Christ Jesus our Lord, who hath enabled me, for that he counted me faithful, putting me into the ministry; Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief. And the grace of our Lord was exceeding abundant with faith and love which is in Christ Jesus. This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.