At Rephidim, Israel have their first encounter with an enemy since escaping from Egypt. The Amalekites, a nomadic tribe of Edomite descent who roam the Sinai peninsula, harass the Israelite caravan, attacking the stragglers (Deuteronomy 25:18). At the Red Sea, God Himself had defended Israel against the Egyptian army; the Israelites hadn’t needed to lift a finger. But from now on, they have to learn to fight for themselves (albeit with God’s help).
Fighting is the job of the younger generation (so Moses commissions Joshua to lead the army into battle); but those who are too old to fight nevertheless have an vital contribution to make, for prayer is the ‘secret weapon’ of God’s people. It is only Moses’ continual intercession (signified by his raised arms) that enables the inexperienced Israelite soldiers to win the day. This kind of prayer is exhausting, and not even Moses can do it all on his own; he needs the help of his two companions.
We too have an enemy, the devil, who will continually harass us on our pilgrimage. And God won’t do everything for us; we need to put on our spiritual armour and go out to do battle against him (Ephesians 6:10-17). Even so, we cannot win the battle in our own strength; we must not forget also to pray (Ephesians 6:18)! In such situations, group prayer is even more powerful and more effective; that is why Paul wanted the believers in Ephesus to pray for him (Ephesians 6:19,20).