Salvation and triangulation

A young man recently was remarking how amazing a women he had found was. She was perfect in every way except that she believed "once saved, always saved." I offered reasons why this approach to salvation was solid, he didn't bite. In his mind this was essentially heresy. Today I read in Hebrews a passage that sounds like a person could lose their salvation. But I had no passion to refute it. It is. I will come across many that talk about God's unending faithfulness, mercy, and grace. I will come across many that make the relationship conditional, contractual. David could fail in almost all respects of his faith and humanity and walk away clean; Saul could try to bolster morale for his troops and honor God in presumption and be banished from his Father's love for the rest of his life (and after?). I submit to the unknowing and am coming to appreciate it. I used to approach the Bible from a "triangulation" perspective - find multiple verses among multiple books (a range in OT and NT preferable) that speak what I want and then feel good about my theology. Now, I realize that there are pieces that will probably never align in my mind and it's OK. Perhaps the fragmentation is partial testimony to God's holiness and his being "other than" us? Who am I?