Only because we must pray Lord, I believe, but help my unbelief do we sin. Saint Paul contrasts faith with sin when he says, Whatever is not of faith is sin. We usually think of sin as the opposite of virtue, and faith as the opposite of doubt. But virtue is a moral term, and doubt is an intellectual term. The opposite of moral virtue is moral vice, and the opposite of intellectual doubt is intellectual belief.
Faith is deeper than either moral virtue or intellectual belief.
Sin is deeper than either moral vice or intellectual doubt. Yes to God with the center of our being, and sin the state of sin as distinct from particular acts of sin is the fundamental No to God with the center of our being. Faith is the opposite of sin. Faith is to sin what light is to darkness. Belief is an intellectual matter.