It wasn't just their exile that caused people to mock God; their behavior fueled the fire even more.
The profanation of God's name here has a twofold cause. First, as we just saw, it was their very exile and suffering that made the nations doubt God's power and faithfulness. They saw 'the people of the LORD' driven from their land and concluded their God wasn't great enough to keep them there. Second, and equally devastating, was Israel's own conduct. Even in exile, or perhaps especially then, their ongoing sinfulness (immorality, dishonesty, etc. as implied by commentators) gave the nations even more reason to mock. They were a terrible advertisement for their God. When people who claim to belong to the Lord live lives that are no different from, or even worse than, non-believers, it gives enemies ample ammunition to blaspheme. As one commentator put it, 'As is the people so is their God.'