Tafsir of Al-Anbiya' 21:75

(And We admitted him into Our mercy) meaning: among the people of Our mercy, that is, We placed him within their group and their number, so the container-relationship (in fi) is metaphorical. Or, it means into Our Paradise, in which case the container-relationship is literal, and "mercy" is a metaphor, as in the Hadith in the two Sahihs where Allah, Mighty and Majestic is He, said to Paradise: "You are My mercy; I show mercy through you to whom I will of My servants." It is also permissible that "mercy" is a metaphor for Prophethood, in which case the container-relationship is also metaphorical. So reflect upon this.

(Indeed, he was of the righteous)

(Those for whom the best has already been preceded from Us) and the sentence is a justification for what preceded it.