You actions build your inherent nature and your inherent nature resides inside your conscience. Hence, your nature determines your conscience. Your conscience influences your faith and your faith, your devotion. Ultimately, its your actions which decide your faith and devotion.
I’m a little confused about this – I would’ve thought that your character determines your actions but Gita says the other way around; and it uses the verb “builds” your character. So the more I think of it the more it makes sense, one may be born with a character but one can always build it through his actions all through his life. That is what is being stressed here – your character develops based on the actions you do and your character influences your decisions on what is right and what is wrong. And this ability leads to faith in something. So the most important thing to do is to do good deeds.