{Ascended not} (ou--aneb(885c)). It is more emphatic than that:
For not David ascended into the heavens. Peter quotes  Ps 110:1 
as proof. No passage in the O.T. is so constantly quoted as
Messianic as this. "St. Peter does not demand belief upon his own
assertion, but he again appeals to the Scriptures, and to words
which could not have received a fulfilment in the case of David"
(Knowling). {Sit thou} (kathou). Late _Koin(825f) form for earlier
kath(8873)o, present middle imperative second singular of
kath(886d)ai.