And Saul built - i. e., of the great stone which they had rolled to kill the oxen and sheep upon, he began to build an altar to Yahweh (see the margin); but he did not finish it (compare 1-Chronicles 27:24), in his haste to pursue the Philistines that night.

Saul built an altar - And this we are informed was the first he had built; Samuel, as prophet had hitherto erected the altars, and Saul thought he had sufficient authority to erect one himself without the prophet, as he once offered sacrifice without him.

And Saul built an altar unto the Lord,.... To offer peace offerings upon, in thankfulness for the victory obtained over his enemies, or sin offerings to make atonement for the sin of the people, perhaps both, however the former:
the same was the first altar that he built unto the Lord; for though he had offered sacrifice at Gilgal, there was an altar ready built for him: or "he began to build"; he laid the first stone of it, and the builders built upon it; so some others say, that he was the first of the kings that built an altar to the Lord (d); others, the first of the judges that built one; though Gideon built one, it was for his own private use, not for all Israel, as this, so R. Isaiah; but Ben Gersom, and so Abarbinel, refer this to the great stone Saul ordered to be rolled to him, and take the sense to be, that that began to be built an altar to the Lord; that was the beginning of one; for he did not now stay to finish it, being eager on his pursuit of the Philistines, as follows.
(d) See Kimchi in loc.

As a thanksgiving for this victory, Saul built an altar to the Lord. לבנות החל אתו, "he began to build it," i.e., he built this altar at the beginning, or as the first altar. This altar was probably not intended to serve as a place of sacrifice, but simply to be a memorial of the presence of God, or the revelation of God which Saul had received in the marvellous victory.