He rebuilt the High Places that his father Hezekiah had torn down. He set up Altars for Baal and made an Asherah pole, exactly the way Ahab king of Israel had done. He worshiped the stars and the heavenly bodies and served them.
He built pagan altars inside the Temple of the Lord — the very place where God had said, "This is where I will put my name." He filled both courtyards of the Temple with altars to the stars.
He sacrificed his own son in fire. He practiced fortune-telling, read omens, and consulted mediums and people who claimed to speak with the dead. He did an enormous amount of evil in God's sight, provoking him to anger.
He even took a carved image of Asherah — an Idol — and placed it inside the Temple. The Temple that God had spoken about to David and Solomon, saying, "In this house, and in Jerusalem, which I have chosen out of all the tribes of Israel, I will put my name forever. And I will not cause Israel to wander from the land I gave their ancestors — if only they are careful to obey everything I commanded them, and all The Law that my servant Moses gave them."
But they did not listen. And Manasseh led them so far astray that they ended up doing more evil than the nations God had destroyed to make room for Israel in the first place.