God criticizes the people for trying to draw near to him without practicing moral actions; a.k.a. for being fake friends. They're super loud about how they're fasting, but all the while they're exploiting their workers.
The problem with these people, says God, is that their acts of humility are only self-interested. They just want some brownie points with God. As it is, God doesn't want people to put on sackcloth and wail in a big show of humility. He actually wants them to share food with the hungry, bring homeless people into their houses, and give clothes to the naked.
If they do all this, they will receive God's favor and be healed from their sins.
The people's light will shine out if they can manage to do all of these things. Oh, but also, they gotta quit accusing other people of sin. Just, generally, no more backbiting. Got it?
Now if they can manage all that, God will guide them and make them strong. Their ruins will be rebuilt, and they'll be praised by future generations for restoring their cities to glory. And also, they'll need to keep the Sabbath holy. Also stop doing self-interested things. (List over. We think)
But, anyway, if they do all that, God will make them "ride on the heights of the earth."