When presented with a complex logic problem that I wasn't sure how to solve, I wrote to myself "I have no friggin' clue, but you'll figure it out. I have faith in Future Me." As it turns out, my faith was well-founded. I puzzled the problem out, solved it, and wrote "Past Me was right to have faith in Present Me."
There is a certain amount of wisdom in having faith in our own abilities, or at least our own potential. There are feats that we are capable of, be they feats of logic, athletics, strength, logistics, or anything else. There are things that we are capable of doing, and we can wisely have faith in our ability to do them.
Naturally, it's possible for this faith to go too far. Overconfidence occasionally occurs. Yet, in my experience, underconfidence is more common, and is sometimes more harmful to our progress. One with too much confidence would make an attempt, fail, and potentially learn from the experience, whereas one with too little confidence would decline making the attempt and would thus learn nothing. So long as the benefit of experience outweighs the consequences of failure, overconfidence is preferable to underconfidence.
Furthermore, it's possible for our competence to grow to match our confidence. Even if we know we cannot succeed currently, it's good to believe that, with enough training, preparation, and strategy, we can succeed at a future time. That faith in our future selves can encourage us to work toward the success we know we can have, which can help us grow in our abilities, even to the point of gaining capabilities we didn't previously have. Even if our Present Selves cannot accomplish a given feat, perhaps our Future Selves can.
Likewise, it's also possible for our competence to shrink to match our lack of confidence in ourselves. The less we believe in ourselves, the less often we try. The less we try, the more our abilities will wane through disuse. Thus, the less confidence we have in ourselves, the less practice and experience we'll give ourselves, and the less competence we'll ultimately have.
The more we try, the better we'll get, and inversely, the less we try, the worse we'll get. So, a large part of success is having enough faith in ourselves to try. There are things we can do, and there are things we can become capable of doing, and it is good and wise to believe that we can do them.