the difference is that the Dhamma is not a theory in a sense of a set of concocted concepts, it’s an expression of direct knowledge and vision, practising which one is able, as at least shown by the sutta narratives, to replicate the Buddha’s realization, one either believes (has faith/confidence/conviction in) this or one doesn’t, and faith (saddha) is a highly praised faculty
And what, bhikkhus, is the wealth of faith? Here, a noble disciple is endowed with faith. He places faith in the enlightenment of the Tathāgata thus: ‘The Blessed One is an arahant, perfectly enlightened, accomplished in true knowledge and conduct, fortunate, knower of the world, unsurpassed trainer of persons to be tamed, teacher of devas and humans, the Enlightened One, the Blessed One.’ This is called the wealth of faith.
Bhikkhus, there are these four streams of merit, streams of the wholesome, nutriments of happiness—heavenly, ripening in happiness, conducive to heaven—that lead to what is wished for, desired, and agreeable, to one’s welfare and happiness. What four?
(1) “Here, a noble disciple possesses unwavering confidence in the Buddha thus: ‘The Blessed One is an arahant, perfectly enlightened, accomplished in true knowledge and conduct, fortunate, knower of the world, unsurpassed trainer of persons to be tamed, teacher of devas and humans, the Enlightened One, the Blessed One.’ This is the first stream of merit….
(2) “Again, a noble disciple possesses unwavering confidence in the Dhamma thus: ‘The Dhamma is well expounded by the Blessed One, directly visible, immediate, inviting one to come and see, applicable, to be personally experienced by the wise.’ This is the second stream of merit….
(3) “Again, a noble disciple possesses unwavering confidence in the Saṅgha thus: ‘The Saṅgha of the Blessed One’s disciples is practicing the good way, practicing the straight way, practicing the true way, practicing the proper way; that is, the four pairs of persons, the eight types of individuals—this Saṅgha of the Blessed One’s disciples is worthy of gifts, worthy of hospitality, worthy of offerings, worthy of reverential salutation, the unsurpassed field of merit for the world.’ This is the third stream of merit….
how one gains faith depends on a person, some need miracles and feats of psychic powers, some need the teaching to resonate with and give answers to their life circumstances, some have kammic predisposition for it, some need personal interaction with an awakened person and so forth
and faith is required because the realization of the Buddhist ideal is a very personal affair, one cannot verify it against some objective preexisting criteria, after all one cannot verify kamma and rebirth
if my own experience is something to go by, then without resort to faith wordlings are only able to acknowledge the first Noble Truth, for some it becomes a sufficient reason for gaining faith in veracity of the rest of the Truths