Faith is not certainty. If you could prove it, faith wouldn't be required. This means doubt isn't the opposite of faith; it's part of its structure. Researchers studying how people hold religious conviction describe one pattern as quest — a readiness to face hard questions without simplifying them, a treatment of doubt as valuable rather than shameful, and an openness to one's own views changing over time. People who hold faith this way are consistently less dogmatic than those who hold it as settled fact. Certainty is often what faith looks like once it has stopped being examined.
Faith is not the same as hope or trust. These sit close together and behave differently. Trust is earned — it's built on a record, and it's reasonable to withdraw it when the record changes. Hope is about outcomes: the belief that things could turn out well. Faith is neither. It's a commitment made where proof isn't available, and held when the evidence has gone quiet or turned against you.
It isn't only religious, and it isn't less than religious. For some people faith is specifically a following of God. For others it's a conviction about what people are capable of, or that the work is worth doing, or that a marriage is worth staying inside during a bad year. The structure is identical in every case: something you've staked your behaviour on that you cannot demonstrate to anyone else.
It only becomes visible when nothing is confirming it. In good conditions, faith and optimism look the same. The difference shows up in the stretch where results haven't arrived, nobody agrees with you, and there's no evidence to point at. What you do then is the actual measure.
That's what faith is — what you keep doing when nothing has yet told you you're right.