Matthew Henry's Commentary on the Whole Bible says this:
David, though a strong believer, needed the help of his friends for the perfecting of what was lacking in his faith; and herein Jonathan was helpful to him, by reminding him of the promise of God, the holy oil wherewith he was anointed, the presence of God with him hitherto, and the many experiences he had had of God’s goodness to him. Thus he strengthened his hands for action, by encouraging his heart, not in the creature, but in God. Jonathan was not in a capacity of doing any thing to strengthen him, but he assured him God would.Even when we cannot do anything personally for our friends, we still can help them find their strength by directing them toward God.
Father God, help us be wise in encouraging one another to find our strength in you and in your word. Send your Holy Spirit among us to help us in this way. In Jesus' name.