For if God’s causal activity is necessary, then God’s causing this universe is necessary, and hence this universe is a necessary being, which is absurd, besides being contrary to the assumptions of typical cosmological arguments. But this objection commits a de re/de dicto fallacy. Consider the argument written out:

1. C1 is God’s causal activity and is a necessary being. (Premise)
2. C1 is God’s causing E. (Premise)
3. Therefore, God’s causing E is a necessary being.
4. Therefore, God necessarily causes E.
5. The F is a necessary being
6. Therefore, necessarily, F exists.

The fallacy is in the last steps 5 and 6.