I believe it's possible to state the most profound theological truths briefly and also to study and teach them infinitely. Think "Eschatology", where we can briefly state that Jesus will return in majesty and glory, but we'll be discussing the program of events until the end of time! The question I've been asked most over the years is whether we can lose our salvation, or to put it in a more positive way, is it Once Saved Always Saved? Here's my brief reply.
I believe in Once Saved Always Saved above all because of the character of God. God gives us eternal life; it is inconceivable that he later withdraws what he has already given us eternally. Philippians 1:6 states, 'being confident of this, that he who began a good work in you will carry it on to completion until the day of Christ Jesus.' This verse points us to the character of God. God initiates salvation, he has begun the good work of salvation in our lives and his character demands he brings that to completion. We did not give ourselves eternal life, God gives it to us and the character of God means that he will bring it to completion. 'It was not Paul who began the good work in Philippi, nor did the Philippians themselves do so by becoming converted. God began it. That strips them ..... of all despondency (about) whether they will keep the faith... It is a question of God's good faith - and there is certainly no question about him.' Karl Barth.
Believers have problems with this doctrine because they see someone who certainly seemed to be a Christian no longing living or believing like one. Have they lost their salvation? Or what about the verses in Hebrews 6? (Answer; read Hebrews 7!). It almost as though we want to achieve assurance of salvation by winning a debate on the issue. If we can satisfactorily explain about the person who looks as though they may have lost salvation or if we can get a watertight interpretation of Hebrews 6 then we'll feel OK. Now these issues constitute part of the very much longer study of this subject and I've certainly got plenty of material that I teach on that. But I'm nervous of an approach that assumes we gain assurance of salvation by just winning a debate to our own satisfaction. Someone else will always tell you that you're wrong anyway!
Consider 2 Peter 1:10 'Therefore, brothers, be all the more diligent to make your calling and election sure...' (ESV). If God calls and God elects then surely our salvation is sure? But really Peter is telling us to live it! I'd suggest that rather then spending too much time trying to win a debate that we live the life and as we do so it helps us to know an assurance of our salvation. We absolutely need to be diligent about the truth, we do well to be diligent in the study of the doctrine but we also need to be diligent in the way we live the life. Didn't Paul say, 'work out your own salvation with fear and trembling. for it is God who works in you...' (Phil 2:12,13)