Exodus 17:15 (ESV) … “And Moses built an altar and called the name of it, The Lord Is My Banner…”
Noah is the first individual of whom it is recorded that he built an altar to the Lord (Gen. 8:20), and this was a common patriarchal practice, particularly in locations where the Lord appeared to them. For instance, when Abram was at Shechem “he built an altar there to the Lord, who had appeared to him” (Gen. 12:7). Although the presentation of sacrifice is not necessarily mentioned at these altars, it would be wrong to suppose that they served only as monuments to the enjoyment of divine fellowship.
The word ‘altar’ is derived from the idea of sacrificial slaughter, and it was only on the basis of sacrifice that fellowship with God was maintained. But that is not to ignore the memorial function of these altars. Abraham gave the name Jehovah-Jireh (‘The Lord will see to it’ or ‘provide’, Gen. 22:14) to the site of the altar on which Isaac had been bound was situated, and when the Lord appeared to Gideon he called the altar he built Jehovah-Shalom (‘The Lord is Peace’, Judg. 6:24).
The name Moses gives here, Jehovah-Nissi, ‘The Lord is my Banner’, refers to the military custom of using a pole (sometimes with a distinctive piece of fabric tied to its end) as a rallying point for troops (Isa. 13:2). At first glance that might seem to favor the interpretation of Moses’ action in verse 11 as one of giving guidance to the army fighting below. But his claim is that it is the Lord himself whose role is like that of a banner. In time of confusion and danger, it is to the Lord that Moses turned in prayer, and it was by his help and power that the victory was won. It is by taking direction from him and relying upon his resources that the Lord’s people will be victorious.