what are some philosophers that believe in certainty that cannot be seen?

Answer 1

I think you are talking about 'faith'.

G.W.F. Hegel

'(1) The Idea of Belief
Φ 528. In the form in which Religion here appears — for it is religion obviously that we are speaking about — as the belief which belongs to the realm of culture, religion does not yet appear as it is truly and completely (an und für sich). It has already come before us in other phases, viz. as the unhappy consciousness, as a form of conscious process with no substantial content in it. So, too, in the case of the ethical substance, it appeared as a belief in the nether-world. But a consciousness of the departed spirit is, strictly speaking, not belief, not the inner essence subsisting in the element of pure consciousness away beyond the actual: there the belief its has itself an immediate existence in the present; its element is the family.'

http://www.marxists.org/reference/archive/hegel/works/ph/phc2b1b.htm

531. For faith its absolute object is a pictorial reflection of the real world with the historical character of that real world.

Φ 531. In the case of belief the aspect of complete being, of being in-and-for-itself, is its absolute object, whose content and character we have already come to know. For it lies in the very notion of belief that this object is nothing else than the real world lifted into the universality of pure consciousness. The articulation of this world, therefore, constitutes the organization belonging to pure universality also, except that the parts in the latter case do not alienate one another when spiritualized, but are complete realities all by themselves, are spirits(5) returned into themselves and self-contained.

The process of their transition from one into the other is, therefore, only for us [who are analysing the process] an alienation of the characteristic nature in which their distinction lies, and only for us, the observers, does it constitute a necessary series; for belief, however, their distinction is a static diversity, and their movement simply a historical fact.

http://www.marxists.org/reference/archive/hegel/li_terms.htm

http://www.marxists.org/reference/archive/hegel/works/ph/phbb.htm

'§ 66

Beyond this point then we need not go: immediate knowledge is to be accepted as a fact. Under these circumstances examination is directed to the field of experience, to a psychological phenomenon. If that be so, we need only note, as the commonest of experiences, that truths which we well know to be results of complicated and highly mediated trains of thought present themselves immediately and without effort to the mind of any man who is familiar with the subject. The mathematician, like everyone who has mastered a particular science, meets any problem with ready-made solutions which presuppose most complicated analyses: and every educated man has a number of general views and maxims which he can muster without trouble, but which can only have sprung from frequent reflection and long experience. The facility we attain in any sort of knowledge, art, or technical expertness, consists in having the particular knowledge or kind of action present to our mind in any case that occurs, even, we may say, immediate in our very limbs, in an outgoing activity. In all these instances, immediacy of knowledge is so far from excluding mediation, that the two things are linked together — immediate knowledge being actually the product and result of mediated knowledge.'

http://www.marxists.org/reference/archive/hegel/works/sl/sl_v.htm#SL66

Answer 2

Blaise Pascal

Answer 3

So many are willing to grant a priori truth...
Huge list.

Answer 4

India saints and sadhus ?:)