Now no more to return to corruption (mēketi mellonta hupostrephein eis diaphthoran). No longer about to return as Lazarus did. Jesus did not die again and so is the first fruits of the resurrection (1Co 15:23; Rom 6:9).
He hath spoken (eirēken). Present perfect active indicative, common way of referring to the permanent utterances of God which are on record in the Scriptures.
The holy and sure blessings of David (ta hosia Daueid ta pista). See 2Sa 7:13. Literally, “the holy things of David the trustworthy things.” He explains “the holy things” at once.