God, and only God is indivisible for the following reasons:

1. God, or the First Cause, cannot be caused, else God would not be the First Cause. Please refer to the Summa Theologica if you have any qualms in this or other regards.

2. The First Cause cannot be a composite, else the First Cause would consist of parts, each of which would have been caused, but as we have said in (1) above, the First Cause cannot be caused. So if the First Cause were to be a composite, it would be composed of two or more causes that were caused, but nothing regarding the First Cause can be caused, therefore, the First Cause cannot be a composite..

3. Since the First Cause cannot be a composite, it cannot be made into a composite by dividing it, because then it would no longer be the First Cause since it would be divisible into parts each of which would require a cause.

4. Therefore, the First Cause, Whom we call God, the Supreme Being, etc., cannot be divided, and is therefore indivisible. Consequently, no other being or entity is indivisible, first because other beings or entities each have generation from the First or secondary causes, and secondly, no other being besides the First Cause is simplicity or unity.