have We raised thee [O Muhammad] as Our Apostle amidst a community [of unbelievers] before whose time [similar] communities have come and gone,52
so that thou might propound to them what We have revealed unto thee: for [in their ignorance] they deny the Most Gracious!53
, Say: "He is my Sustainer. There is no deity save Him. In Him have I placed my trust, and unto Him is my recourse!"
Most of the commentators explain the "thus" or "thus it is" (kadhalika) as referring to the earlier prophets, namely, "Thus, [or "in like manner"] as We sent prophets before thee, O Muhammad, We have now sent thee...", etc. It seems to me, however, that this speculative interpolation is unnecessary, and that the adverb "thus" connects directly with the preceding statement that God "guides unto Himself all who turn unto Him": in other words, the "thus" qualifies Muhammad's mission as an instrument of God's guidance. (This is, apparently, how Tabari understands the above phrase.)
Lit., "before whom [other] communities have passed away": an indirect reference to the continuity of prophetic revelation before and up to the time of the Last Prophet Muhammad (Zamakhshari, Razi). The interpolation of the words "of unbelievers" is based on Ibn Kathir's commentary on this verse, whereas my rendering of arsalnaka (lit., "We have sent thee") as "We have raised thee as Our Apostle" is necessitated, in English, by the subsequent preposition "amidst".
I.e., by refusing to acknowledge His existence, or by rejecting His guidance, or by ascribing divine qualities to other beings or forces side by side with Him.