The temple was the earthly place where the Lord’s glory dwelled among his people. This was one of the greatest blessings of the covenant.
But this blessing depended on the people being holy (set apart for God and living in obedience to him). When they filled the temple with sin and idolatry, they defiled it.
Because of their unfaithfulness, the Lord left his house. Once his presence departed, the temple and the city were left unprotected. They became vulnerable to the coming attack by the Babylonians.