Some theological issues can be studied; others have to be wrestled with. The question of whether or not a man or woman who is saved will always be saved falls into the latter category.
We have two seemingly contradictory pictures in the Bible. On the one hand there are the commands to watch out lest we fall: "Therefore, dear friends, since you have been forewarned, be on your guard so that you may not be carried away by the error of the lawless and fall from your secure position." (2 Peter 3v17) or "Work out your salvation with fear and trembling" (Philippians 2v12). And there are example in the Bible, and from the church throughout the ages, of people who having once professed to be Christians, and seemed to walk with God, then turned away from it all.
And yet on the other hand we have the statement of Jesus: "And this is the will of him who sent me, that I shall lose none of all those he has given me, but raise them up at the last day. For my Father’s will is that everyone who looks to the Son and believes in him shall have eternal life, and I will raise them up at the last day." (John 6v39-40).
We find a promise of Christ, that he will lose none of those given to him by the Father, seemingly at odds with both other verses in the Bible and our experience of seeing people fall from faith. This requires us to study further for a logic principle of the infallibility of the Bible is that Scripture cannot contradict Scripture.