Exodus 17 records the first battle where the Israelites took part in the actual fighting. This battle brought about a great victory. Moses built an altar and called its name YHWH Nissi, or "The-Lord-Is-My-Banner." The word "banner" means "sign." Together we may understand this victory as a sign for the Israelites that God was truly with them. The altar became a landmark for the Israelites as they traveled through the wilderness towards the Promised Land.
If we read through the wilderness account, we realize from the crossing the Red Sea onwards, there were many places that were significant to the Israelites' experience. For example, when the Israelites reached the Wilderness of Shur, they found no water. They went to another place called Marah, which means "bitter." The water could not be drunk, for it was bitter. But God made the water sweet. Later the Israelites reached a place called Elim which means "sweet" (Exod 15:22-27). Each place marked a memory of God's work in their wilderness life.
Likewise, the altar Moses built was a reminder to the Israelites. They were to have this memory etched into their hearts: God was with them. God is their victory. Moses wanted to remind them, too, that victory was theirs because they had followed the voice of God. The Israelites had shown few complaints this time. They had not said to God, "You have brought us out here to die!" Rather, they had silently followed Moses' and Joshua's example of faith. The united effort had brought about a united victory for the people of God.
The apostles also experienced such a victory when, as they joined together "with one accord in one place" on the Day of Pentecost, "they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance" (Acts 2:1-4). This was the evidence of God's promise fulfilled: "You shall receive power when the Holy Spirit has come upon you; and you shall be witnesses to Me in Jerusalem, and in all Judea and Samaria, and to the end of the earth" (Acts 1:8). The Holy Spirit bore witness that the Lord's presence was with them, for Jesus had said to them, "I am with you always, even to the end of the age" (Mt 28:20). When we receive the Holy Spirit today, it is also etched in our hearts: the Lord is with us. And we, too, can proclaim in worship and in witness, "The Lord is my Banner" to the ends of the earth, and to the end of the age.