Jamieson, Fausset and Brown on Amos 3:14

14. That--rather, "since," or "for." This verse is not, as English Version translates, the thing which the witnesses cited are to "testify" (Am 3:13), but the reason why God calls on the heathen to witness Samaria's guilt; namely, in order to justify the punishment which He declares He will inflict.

I will also visit ... Beth-el--the golden calves which were the source of all "the transgressions of Israel" (1Ki 12:32; 13:2; 2Ki 23:15, 16), though Israel thought that by them their transgressions were atoned for and God's favor secured.

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 (2Ch 34:5, compare with 1Ki 13:2; Ho 8:11; 10:1).