"I, I am he who comforts you...I am Yahweh your God, saying to Zion, 'You are my people.'" (Isaiah 51:12, 15a, 16b)
True courage comes not from our own bravado but from the promises of God. If we have to fend for ourselves, however strong we may be, we are doomed. But if God is watching over us, we can face anything, even with quiet good cheer. Isaiah is putting steel in our backbones by arguing that our hope is as secure as the existence of God: "I, I am he who comforts you." This is what God most wants us to grasp...
God forms an unbreakable bond with his people and gives them firm promises. The conversation goes like this:
GOD: "I am your God. You are my people. I will bless you."
SINNERS: "But at our best, we barely believe you."
GOD: "True. But I will bless you."
SINNERS: "But we don't deserve you."
GOD: "More than you know! But I will bless you."
SINNERS: "But we don't live up to this. We're cowards."
GOD: "You are. But I will bless you."
SINNERS: "But we're so entrenched in this world, we'll never change."
GOD: "Not true. You are Zion, the eternal city of God. I will bless you accordingly, and everything will change."
(Isaiah, p. 345, Ray Ortlund, Jr.)