That in the day that I shall visit the transgressions of Israel upon him I will also visit the altars of Bethel: and the horns of the altar shall be cut off, and fall to the ground.
That - rather, Since or For. This verse is not as the English version translates, the thing which the witnesses cited are to "testify" (Amos 3:13), but the reason why God calls on the pagan to witness Samaria's guilt-namely, in order to justify the punishment which He here declares He will inflict.
I will also visit the altars of Beth-el - the golden calves, which were the source of all "the transgression of Israel" (cf. 1 Kings 12:32; 1 Kings 13:2, where the man of God denounces the altar of Bethel as doomed to be defiled by Josiah's, offering upon it "the priests of the high places." Accordingly, Josiah "brake down both that altar and the high place, and burned and stamped the high place small to powder;" 2 Kings 23:15-16); yet Israel thought that by them their transgressions were atoned for, and God's favour secured.
And the horns of the altar - which used to be sprinkled with the blood of victims. They were horn-like projecting points at the corners of ancient altars. The singular, "altar," refers to the great altar erected by Jeroboam to the calves. The "altars," plural, refer to the lesser ones made in imitation of the great one (2 Chronicles 34:5: cf. with 1 Kings 13:2; Hosea 8:11). It is the tendency of heresy to spread "like a cancer" (2 Timothy 2:17). Beginning with as little deviation as possible from the truth, in order the more readily to deceive, from the one altar, which professed to honour the unity of God, they in course of time passed on to make many altars, and to diverge more and more from the divine truth (2 Kings 10:1).