A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
1 Chronicles 21:1
ch1 21:1DAVID SINS IN NUMBERING THE PEOPLE. (Ch1 21:1-13)
Satan stood up against Israel--God, by withdrawing His grace at this time from David (see on Sa2 24:1), permitted the tempter to prevail over him. As the result of this successful temptation was the entail of a heavy calamity as a punishment from God upon the people, it might be said that "Satan stood up against Israel."
number Israel--In the act of taking the census of a people, there is not only no evil, but much utility. But numbering Israel--that people who were to become as the stars for multitude, implying a distrust of the divine promise, was a sin; and though it had been done with impunity in the time of Moses, at that enumeration each of the people had contributed "half a shekel towards the building of the tabernacle," that there might be no plague among them when he numbered them (Exo 30:12). Hence the numbering of that people was in itself regarded as an undertaking by which the anger of God could be easily aroused; but when the arrangements were made by Moses for the taking of the census, God was not angry because the people were numbered for the express purpose of the tax for the sanctuary, and the money which was thus collected ("the atonement money," Exo 30:16) appeased Him. Everything depended, therefore, upon the design of the census [BERTHEAU]. The sin of David numbering the people consisted in its being either to gratify his pride to ascertain the number of warriors he could muster for some meditated plan of conquest; or, perhaps, more likely still, to institute a regular and permanent system of taxation, which he deemed necessary to provide an adequate establishment for the monarchy, but which was regarded as a tyrannical and oppressive exaction--an innovation on the liberty of the people--a departure from ancient usage unbecoming a king of Israel.
1 Chronicles 21:3
ch1 21:3why will he be a cause of trespass to Israel?--or bring an occasion of punishment on Israel. In Hebrew, the word "sin" is often used synonymously with the punishment of sin. In the course of Providence, the people frequently suffer for the misconduct of their rulers.
1 Chronicles 21:5
ch1 21:5Joab gave the sum of the number of the children of Israel--It amounted to one million one hundred thousand men in Israel, capable of bearing arms, inclusive of the three hundred thousand military (Ch1 27:1-9), which, being already enlisted in the royal service, were not reckoned (Sa2 24:9), and to four hundred seventy thousand men in Judah, omitting thirty thousand which formed an army of observation stationed on the Philistine frontier (Sa2 6:1). So large a population at this early period, considering the limited extent of the country and comparing it with the earlier census (Num. 26:1-65), is a striking proof of the fulfilment of the promise (Gen 15:5).
1 Chronicles 21:6
ch1 21:6Levi and Benjamin counted he not--If this census was ordered with a view to the imposition of taxes, this alone would account for Levi, who were not warriors (Ch1 21:5), not being numbered (see on Num 1:47-54). The population of Benjamin had been taken (see on Ch1 7:6-11), and the register preserved in the archives of that tribe. This, however, was taken on another occasion, and by other agency than that of Joab. The non-numbering of these two tribes might have originated in the special and gracious providence of God, partly because Levi was devoted to His service, and Benjamin had become the least of all the tribes (Jdg. 21:1-25); and partly because God foresaw that they would remain faithful to the house of David in the division of the tribes, and therefore He would not have them diminished [POOLE]. From the course followed in this survey (see on Sa2 24:4-8), it would appear that Judah and Benjamin were the last tribes that were to be visited; and that, after the census in Judah had been finished, Joab, before entering on that of Benjamin, had to return to Jerusalem, where the king, now sensible of his great error, gave orders to stop all further proceedings in the business. Not only the remonstrance of Joab at the first, but his slow progress in the survey (Sa2 24:8) showed the strong repugnance and even horror of the old general at this unconstitutional measure.
1 Chronicles 21:9
ch1 21:9the Lord spake unto Gad, David's seer--Although David was himself endowed with a prophetic gift, yet, in matters relating to himself or his kingdom, he was in the habit of consulting the Lord through the medium of the priests; and when he failed to do so, a prophet was sent on extraordinary occasions to admonish or chastise him. Gad, a private friend, was occasionally employed as the bearer of these prophetic messages.
1 Chronicles 21:11
ch1 21:11Choose thee, &c.--To the three evils these correspond in beautiful agreement: three years, three months, three days [BERTHEAU]. (See on Sa2 24:13).
1 Chronicles 21:13
ch1 21:13let me fall now into the hand of the Lord . . . let me not fall into the hand of man--Experience had taught him that human passion and vengeance had no bounds, whereas our wise and gracious Father in heaven knows the kind, and regulates the extent, of chastisement which every one needs.
1 Chronicles 21:14
ch1 21:14So the Lord . . . sent an angel unto Jerusalem to destroy it--The infliction only of the pestilence is here noticed, without any account of its duration or its ravages, while a minute description is given of the visible appearance and menacing attitude of the destroying angel.
1 Chronicles 21:15
ch1 21:15stood by the threshing-floor of Ornan the Jebusite--Ornan was probably his Hebrew or Jewish, Araunah his Jebusite or Canaanitish, name. Whether he was the old king of Jebus, as that title is given to him (Sa2 24:23), or not, he had been converted to the worship of the true God, and was possessed both of property and influence.
1 Chronicles 21:16
ch1 21:16David and the elders . . . clothed in sackcloth, fell upon their faces--They appeared in the garb and assumed the attitude of humble penitents, confessing their sins, and deprecating the wrath of God.
1 Chronicles 21:18
ch1 21:18HE BUILDS AN ALTAR. (Ch1 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 (Sa2 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.
1 Chronicles 21:20
ch1 21:20Ornan was threshing wheat--If the census was entered upon in autumn, the beginning of the civil year, the nine and a half months it occupied would end at wheat harvest. The common way of threshing corn is by spreading it out on a high level area, and driving backwards and forwards upon it two oxen harnessed to a clumsy sledge with three rollers and some sharp spikes. The driver sits on his knees on the box, while another person is employed in drawing back the straw and separating it from the grain underneath. By this operation the chaff is very much chopped, and the grain threshed out.
1 Chronicles 21:23
ch1 21:23I give thee . . . the threshing instruments for wood--that is, to burn the sacrifice of the oxen. Very little real import--the haste and the value of the present offered--can be understood in this country. The offering was made for instant use. Ornan, hereby hoping to terminate the pestilence without a moment's delay, "gave all," oxen, the large threshing machine, and the wheat.
1 Chronicles 21:25
ch1 21:25David gave . . . for the place six hundred shekels of gold--At first he bought only the cattle and the threshing instruments, for which he paid fifty shekels of silver (Sa2 24:24); afterwards he purchased the whole property, Mount Moriah, on which the future temple stood. High in the center of the mountain platform rises a remarkable rock, now covered by the dome of "the Sakrah." It is irregular in its form, and measures about sixty feet in one direction and fifty feet in the other. It is the natural surface of Mount Moriah and is thought by many to be the rock of the threshing-floor of Araunah, selected by David, and continued by Solomon and Zerubbabel as "the unhewn stone" on which to build the altar [BARTLETT, Walks about Jerusalem; STANLEY].
1 Chronicles 21:26
ch1 21:26David built there an altar--He went in procession with his leading men from the royal palace, down Mount Zion, and through the intervening city. Although he had plenty of space on his own property, he was commanded, under peremptory direction, to go a considerable distance from his home, up Mount Moriah, to erect an altar on premises which he had to buy. It was on or close to the spot where Abraham had offered up Isaac.
answered him by fire from heaven--(See Lev 9:24; Kg1 18:21-23; Kg2 1:12; Ch2 7:1).
1 Chronicles 21:28
ch1 21:28when David saw that the Lord had answered him . . ., he sacrificed there--or, "he continued to sacrifice there." Perceiving his sacrifice was acceptable, he proceeded to make additional offerings there, and seek favor by prayer and expiatory rites; for the dread of the menacing angel destroying Jerusalem while he was absent in the center of worship at Gibeon, especially reverence for the Divine Being, led him to continue his adorations in that place which God (Ch2 3:1) had hallowed by the tokens of His presence and gracious acceptance.