We are witnesses (hēmeis esōmen martureōs). As in Act 2:32.
Things (rhēmatōn). Literally, sayings, but like the Hebrew dabhar for “word” it is here used for “things.”
And so is the Holy Ghost (kai to pneuma to hagion). The word for “is” (estin) is not in the Greek, but this is plainly the meaning. Peter claims the witness of the Holy Spirit to the raising of Jesus Christ, God’s Son, by the Father.