The Story
Moses was up on Mount Sinai with God for forty days and forty nights. Down below, the people grew tired of waiting. Moses has been gone so long, they said — we do not know what has become of him. So they crowded around Aaron and made their wicked request: "Up, make us gods, which shall go before us".
Aaron told them to bring their golden earrings, and they brought piles of gold. Aaron melted the gold and shaped it into a calf — a young cow made of gold. Then the people shouted, "These be thy gods, O Israel", saying that this golden calf had brought them out of Egypt! Aaron built an altar before it, and the next day the people offered sacrifices, ate and drank, and played and danced around their idol.
Up on the mountain, God told Moses what the people had done. God was very angry, and He spoke of destroying the wicked people and making a great nation from Moses instead. But Moses loved the people, and he pleaded with God for them. He reminded God of His promises to Abraham, Isaac, and Jacob, and of how He had brought Israel out of Egypt with great power. And God listened to Moses: "the LORD repented of the evil which he thought to do unto his people".
Moses came down the mountain carrying the two stone tables of God's law, written by God's own finger. He heard singing, and he saw the calf and the dancing — and in his anger he threw down the tables and broke them at the foot of the mountain. He took the golden calf, burned it, ground it to powder, and destroyed it completely. He asked Aaron how he could have led the people into such a great sin.
Then Moses stood in the gate of the camp and called out for everyone who was on the LORD's side to come to him — that is our memory verse — and the men of Levi came and stood with him.
And still Moses was not done praying. The next day he went back up the mountain to plead for the people once more. He loved them so much that he prayed, "blot me, I pray thee, out of thy book which thou hast written" — he would rather lose his own place in God's book than see his people lost. God did forgive His people, and He promised to keep leading them toward the promised land. The people's sin was great — but God's mercy was greater.