18 Then the angel of Yahweh commanded Gad to tell David that David should go up, and raise an altar to Yahweh in the threshing floor of Ornan the Jebusite.
*Minor differences ignored. Grouped by changes, with first version listed as example.
It has been observed that it is only in books of a late period that Angels are brought forward as intermediaries between God and the prophets. This, no doubt, is true; and it is certainly unlikely that the records, from which the author of Chronicles drew, spoke of Gad as receiving his knowledge of God's will from an angel. The touch may be regarded as coming from the writer of Chronicles himself, who expresses the fact related by his authorities in the language of his own day (see Zac 1:9, Zac 1:14, Zac 1:19; Zac 2:3; Zac 4:1; Zac 5:5; etc.); language, however, which we are not to regard as rhetorical, but as strictly in accordance with truth, since Angels were doubtless employed as media between God and the prophet as much in the time of David as in that of Zechariah.
HE BUILDS AN ALTAR. (1-Chronicles 21:18-30)
the angel of the Lord commanded Gad to say--The order about the erection of an altar, as well as the indication of its site, is described (2-Samuel 24:18) as brought directly by Gad. Here we are informed of the quarter whence the prophet got his commission. It is only in the later stages of Israel's history that we find angels employed in communicating the divine will to the prophets.
Set up an altar, &c. - The commanding of David to build an altar, was a blessed token of reconciliation. For if God had been pleased to kill him, he would not have commanded, because he would not have accepted a sacrifice at his hands.
*More commentary available at chapter level.