God should exist on his own, as a matter of right, and not in the gaps in human knowledge.

The argument from design cannot account for God himself. If God must have created the universe, because the universe is so complex, then God himself must be even more complex than the universe, and then you have to explain how he came about.

If you say he just is, he is a brute fact, then a less complex universe has a higher chance of just being, because a complex universe is easier to explain than a complex universe coupled with an even more complex God.

And it is a huge leap to go from saying there is God to saying that the God is the one in the bible, who is a very weird character for a God.