I would argue that it's a little bit of both. The aforementioned religious background has taught me that certain things are true of all people. For instance, people are inherently valuable. They can neither increase nor diminish this value, even as they are not responsible for its being true of them in the first place. But there are other qualities of personal identity that are much more fluid (and thankfully so) like whether one is merciful, selfish, or creative. As the Dumbledore quote on my new cell phone case says, "It is our choices that show who we truly are, far more than our abilities." Even if we can't assign a value to ourselves, we can choose whether we will act selflessly or selfishly, and such choices made consistently over time cement themselves into personal characteristics (although perhaps "cement" is the wrong word to use about something we are always able to change, if with some difficulty).
That was my first point. This is my second (and slightly more important) one.
Everyone has dreams and ideas about where they would like to be in the future or what they would like to do with their lives. And everyone has failed, at some point or another, to make their dreams a reality. The thing is, we often sabotage our own dreams because we don't think we're a certain kind of person. Perhaps you want to have children, but you're concerned that you're not a very paternal or maternal person. You've always secretly been convinced that babies are ugly little aliens, no matter how cute everyone says they are, and you're not sure how to interact with kids, so you've convinced yourself that you really shouldn't parent any of your own.
In my case, I've always told myself that I'm not very creative. It's true that I'm all thumbs with a paintbrush in hand, and if you give me a sketchbook, I'll probably fill it with quotes written in beautiful penmanship that slant up on the one side because I can't write in straight lines. I've accepted this negative definition of myself ("not creative") without argument and almost doggedly, in spite of short stories, poetry, blog posts, jewelry, ceilings, sewing projects, and various crafts to the contrary. Which is why I've been haunted on and off by a note from my friend David since he gave it to me in Brazil three years ago. Alongside humorous observations about Oxford commas, he made the passing comment that I am a creative person. Who, me?
Clearly my view of myself has not thus far prevented me from actually doing creative things, but how many people really have decided not to have children because of how they've identified themselves? What we say to ourselves about who we are plays a distinct role in our willingness to take risks and achieve the things we dream about.
In sum, say nice things to yourself, and if they aren't already true of who you are, then make the appropriate choices so that they become true of you. Otherwise, you might miss out on a really cool life, and that would be super lame.