Jehovah Nissi is translated as "The Lord My Banner". Nes is a Hebrew word that is translated as "banner", which is the insignia under which an army would fight. We might think of it as an identifying flag, the sight of which would give courage and hope to the soldiers.
It is used only once in the Old Testament, in Exodus 17:15. The Israelites had just finished fighting Amalek and his army. While the men fought, Moses stationed himself on the top of a hill with the staff of God in his hand. As long as Moses held his hand up, Israel prevailed, but if he left his hand down, Amalek prevailed. Eventually, his arms were so tired that he couldn't hold his arms up any longer, so Aaron and Hur dragged a big stone over for Moses to sit on and they stood on either side of him, holding up his hands. The two men held his arms up until after sunset, and Joshua and the army won the battle.
Afterwards, Moses built an altar and named it "The Lord is My Banner", or Jehovah Nissi. While the men were fighting, they could look to the top of the hill and see Moses with his hands raised, holding the staff of God. It would have been easy to think that Moses and his upraised hands were the banner over them, the rallying point and the source of their hope and courage. After the battle, Moses, the most humble man who ever lived, built an altar to God and named it Jehovah Nissi. In doing that, he made it very clear that neither he nor his upraised arms were the banner over them. It was Almighty God alone who was their source of hope, their source of strength, their source of courage. It was God, and God alone, who had delivered them, and it was to Him alone that they should look for help in time of trouble. He is the banner over us, under which we work and do battle.
We would do well to remember this important lesson. No matter how it looks, no matter the circumstances, any victory that is achieved is not because of our ability, because of how well we obeyed, nor because of the ones who helped up to accomplish the task. All the credit, all the glory, must go to God alone. When we know God as Jehovah Nissi, we look to Him for our hope and our courage. We look to Him in the midst of battle, and He is the banner to which we look when our faith falters. As long as the flag still flies, an army is not defeated, and we serve a God whose banner never falls.
How, then, do we hallow the name of Jehovah Nissi? We must be careful to honor Him as Moses did, giving Him all the credit, all the glory, and, especially in victory, pointing toward our Lord and away from ourselves. He is our banner. Let's be sure we keep our eyes fixed on Him.
"He has brought me to his banquet hall,
and his banner over me is love"
Song of Solomon 2:4