I know also, my God, that you try the heart, and have pleasure in uprightness. As for me, in the uprightness of my heart I have willingly offered all these things: and now have I seen with joy your people, that are present here, offer willingly to you.
So David is, has told of his own commitment and now he's challenging them for a commitment of giving to God. "This is what I have done. This is what I propose to do. Now who of you are willing to offer your service unto the Lord?"