“Now go, lead the people.” This points to Moses’ sixth trip down Mount Sinai, which is not expressly stated in the Bible but which we know happened because Moses shows up with the people who are down at the base of Mount Sinai.
[For more information on Moses’ seven trips up Mount Sinai, see commentary on Exod. 19:3.]
“my angel will go before you.” God, in His mercy, apparently to some extent restores the situation that existed before Israel sinned: God’s angel will go ahead of Israel and will lead them into the Promised Land (Exod. 23:20-23). However, God Himself would not go with them because their sin might cause Him to destroy them (Exod. 33:3).
“in the day when I visit, I will visit their sin upon them.” God forgave the sin of the Israelites, but it still had consequences. In the war between Good and Evil, between God and the Devil, when God’s people sin it opens up the door for the Devil to afflict them, and he did. The text uses the common idiom of permission and words the text as if God did the punishing.
When God “visited” someone, He intervened in their life, and He could intervene for the better or for the worse. God is a righteous God, and He holds people accountable for their actions. Although the word “visit” could be translated “punish” here, and many versions do that, the word “visit” shows that God does not just “punish,” He visits and evaluates the situation and then acts accordingly. In this case, however, “I will visit their sin upon them” means that God will punish people for their sin.
[For more on “visit,” see commentary on Exodus 3:16 and 20:5. For more on the idiom of permission, see commentary on Exod. 4:21.]