The cliché "Once Saved, Always Saved" is true, but the real question is, Have you been saved? How do you know if you have been saved? "I prayed the 'sinner's prayer' when I was six" or "I asked Jesus into my heart when I was twelve" are not satisfactory answers. In fact, there is nothing biblical pertaining to either one. While a person's initial profession may be evidence of genuine conversion, nevertheless it is insufficient evidence. Jesus taught that many people would respond positively to the Gospel, only later to demonstrate their unregenerate state by falling away (Matt. 13).
Evidence of your justification must be looked for beyond the moment of your suspected conversion. Peter wrote, "be all the more diligent to make certain about His calling and choosing you" (2 Pet. 1:10). In other words, grow in assurance of your salvation by increasing in virtue, knowledge, self-control, perseverance, godliness, brotherly kindness, and love (2 Pet. 1:5-11). James wrote, "faith without works is useless" (Jam. 2:20). In other words, good works always accompany genuine saving faith (Jam. 2:14-26). Paul wrote, "walk by the Spirit, and you will not carry out the desire of the flesh" (Gal. 5:16). In other words, it is those who are led by the Spirit—those who kill sin and grow in personal holiness by the Spirit's power—who are sons of God (Rom. 8:13-14).
If you are assured of your justification, then you can be assured that God will not allow anything to snatch you from His hand (John 10:28-29) or allow anything in Heaven or on Earth to separate you from His love for you in Christ Jesus (Rom. 8:38-39). He will keep you from falling away (Jude 1:24), protect you from the evil one (1 John 5:18), and complete the redemptive work that He first began in you (Phil. 1:6). The cliché "Once Saved, Always Saved" is only true if the individual has truly been saved to begin with. Otherwise, they are false converts and their falling away makes them apostates. Genuine believers cannot become apostates (1 John 2:19).