Ahaz went to Damascus - He had received so much help on the defeat of Rezin, that he went to Damascus to meet the king of Assyria, and render him thanks.
Ahaz sent to Urijah the priest the fashion of the altar - This was some idolatrous altar, the shape and workmanship of which pleased Ahaz so well that he determined to have one like it at Jerusalem. For this he had no Divine authority, and the compliance of Urijah was both mean and sinful. That Ahaz did this for an idolatrous purpose, is evident from 2 Chronicles 28:21-25; : "For he sacrificed to the gods of Damascus; - and he said, Because the gods of the kings of Syria help them, I will sacrifice to them, that they may help me. And he made high places to burn incense to other gods in every city of Judah."
And saw an altar - Rather, “The altar,” i. e. an Assyrian altar, and connected with that formal recognition of the Assyrian deities which the Ninevite monarchs appear to have required of all the nations whom they received into their empire.
The fashion of the altar - Assyrian altars were not very elaborate, but they were very different from the Jewish. They were comparatively small, and scarcely suited for “whole burnt-offerings.” One type was square, about half the height of a man, and ornamented round the top with a sort of battlement. Another had a triangular base and a circular top consisting of a single flat stone. A third was a sort of portable stand, narrow, and about the height of a man. This last was of the kind which the kings took with them in their expeditions.