Tafsir of Al-An'am 6:155

(And this is a Book We have revealed), meaning the Qur’an, the commands and prohibitions of which have been recited to you. It is a (Book) of immense status, the measure of which cannot be estimated. (We have revealed it) by means of the Trustworthy Spirit (Gabriel), containing the benefits of religious and worldly arts, a portion of which has been detailed to you. This sentence is an adjective for "Book."

His statement, may He be glorified, (blessed)—that is, possessing abundant good in both religion and worldly life—is another adjective. The first adjective was placed before the second, even though the former is not sufficient [in description], because the discourse is addressed to those who deny the revelation. It is also permissible that this and the preceding [adjective] are both predicates for the demonstrative pronoun.

The fa (particle) in His saying, (so follow it), is for ordering what follows it upon what precedes it. For the greatness of the Book’s status in itself and its [blessed] attribute necessitates following it—that is, act upon what is within it or comply with its commands. (And fear) contradicting it or [fearing] its prohibitions, (so that you may receive mercy)—meaning, so that you may be shown mercy as a reward for that. It is also said: The intent is "fear with the hope of mercy," or "fear so that the purpose of the piety may be the mercy of Allah the Exalted."