In his journal for September 1, 1849, we find the following record of a sermon Brigham Young himself delivered, showing the absolute necessity of divine revelation for us to truly know the reality of God and the truth of the Gospel:
I look upon my own
existence to the li[gh]t within me, if I have talent given me I shall venture
to tell them in part—for an exp[anatio]n on this point wo[uld] require many
worlds to convey the idea unless h[eav]n[l]y words r given, it wo[ul]d require
vol[umes] to explain—there is a Sp[irit] in every human being & living
creature [and] the insp[iratio]n of the Al[might]y giveth it understanding,
without that, we co[ul]ld not convey it to ano[the]r man—consequently my
doctrine is [that] absolutely no man or woman can understand the things of God
unless the Al[mighty] giveth understanding—the whole of the Gospel is founded
on that prin[ciple that] we must [k]no[w] by the rev[elatio]n of J[esus] C[hrist]—[To[
recollect[,] I apply a Sp[irit]—yet philosophy applies life to all. (George D.
Smith, ed., Brigham Young, Colonizer of the American West: Diaries and
Office Journals, 1832-1871, 2 vols. [Salt Lake City: Signature Books,
2021], 1:160)