# General Information:

Yahweh speaks of the false security of peace that the prophets spoke as if they had built a poorly-constructed wall and covered it over with white paint in order to make it look good. 

# Because of this

This refers to the prophets reporting false visions to the people and telling them lies.

# they led my people astray

Yahweh speaks of the prophets as if they had led the people away from the path they should have been walking.

# they are building a wall that they will paint with whitewash

Here "a wall" stands for peace and security that the false prophets told the people Yahweh was promising to give them. Whitewash is a white liquid mixture or paint used to cover up impurities and make surfaces white.