Nietsche may be alluding to formulas of Acquinas's theology, where one reads that "the science concerning the soul is most certain" and "mind is said to know nothing better than what is in itself". Acquinas however refers to the utmost certainty about soul's own existence and content (the idea goes back to Augustine, and was famously appropriated by Descartes), whereas Nietsche has in mind a "new and deeper meaning".
Earlier in the chapter What is Noble he mentions that "the multitude... have on their part learnt great reverence--reverence for "great men"". Alas, "SUCCESS has always been the greatest liar... the "work" of the artist, of the philosopher, only invents him who has created it, is REPUTED to have created it; the "great men," as they are reverenced, are poor little fictions composed afterwards". The reverence of works is therefore exposed as reverence of fakes. But by reverencing works artists and scholars betray a real longing, longing for nobleness. "But this very NEED of nobleness is radically different from the needs of the noble soul itself, and is in fact the eloquent and dangerous sign of the lack thereof".
And Nietsche has a remedy for this self-alienation. To satisfy the needs of the noble soul he suggests turning to the most fundamental certainty of all, the one from the "old religious formula", the certainty of self. "It is not the works, but the BELIEF which is here decisive and determines the order of rank--to employ once more an old religious formula with a new and deeper meaning--it is some fundamental certainty which a noble soul has about itself, something which is not to be sought, is not to be found, and perhaps, also, is not to be lost.--THE NOBLE SOUL HAS REVERENCE FOR ITSELF.--"