{Unto which} (Tini). "To which individual angel." As a
class angels are called sons of God (Elohim) ( Ps 29:1 ), but no
single angel is called God's Son like the Messiah in  Ps 2:7 .
Dods takes "have I begotten thee" (gegenn(886b)a se, perfect active
indicative of genna(935c)) to refer to the resurrection and
ascension while others refer it to the incarnation. {And again}
(kai palin). This quotation is from  2Sa 7:14 . Note the use of
eis in the predicate with the sense of "as" like the Hebrew
(LXX idiom), not preserved in the English. See  Mt 19:5  Lu
2:34 . Like Old English "to" or "for." See  2Co 6:18  Re 21:7 
for the same passage applied to relation between God and
Christians while here it is treated as Messianic.