Radak

Let me declare it for a decree: – This is the utterance of David, who says, I will make this declaration a decree and rule. And what is the declaration? That The Lord said unto me, Thou art My son: – and the kingdom has come to me from Him, and therefore let no man stir up strife against it, for the Lord hath chosen me for a son, as He said to Samuel (1 Sam. 16:1): “For I have provided among his sons a king for Myself.” It is as though to say, “This king is Mine and he is My son and servant and obeys Me” – for everyone who is obedient in the service of God He calls His son, just as a son obeys his father and is ready for his service. And so (in the verse) “ye are sons of the Lord your God” (Deut. 14:1), and “I will be his Father and he shall be My son” (2 Sam. 7:14); and it says (Hos. 2:1), “the sons of the living God.” This day have I begotten thee: – The day when he was anointed was the day God took him to Himself for a son, as it says (1 Sam. 16:1), “to Me a king.” And he says, This day have I begotten thee, for on that day there was born in him the spirit of God, as is written (1 Sam. 16:13), “And the spirit of the Lord came mightily upon David from that day and onward.” And from that day onward he gave expression to songs and psalms by the Holy Spirit, which was born in him and imparted to him by God. Moreover, the spirit of might was added to him from that day onward; and so it says (Deut. 32:18), “Of the rock that begat thee thou art unmindful” which made thee great and put wisdom in thine heart and intelligence, in the day of the station at Mount Sinai. And so it says (ibid. 6), “Is not He thy Father that hath bought thee?” For if he had said begotten thee, bought thee in reference to (the act of) creation, this would have been no more true of Israel than the rest of men. Do you not see what he says, “Do ye thus requite the Lord?” (ibid.), meaning, “This is the requital you make Him for all the good He has done to you! Surely He is thy Father who bought thee!” And he says, “Of the rock that begat thee thou art unmindful and hast forgotten the God that gave thee birth” (ibid. 18), just as he says thy Father, for He is to you as a father, and the one that gave thee birth – that is, the mother.