The Mereological Argument for Classical Theism

Graham Oppy has presented a mereological argument for pantheism.  What I want to explore is whether mereological realism can establish classical theism.

1. Every physical composite thing is composed of parts. (Definition)

2. The parts of a composite thing are either material or immaterial. (Definition)

3. If the parts of a composite thing are material, then those parts can be reduced to fundamental particles. (Premise)

4. Fundamental particles are either necessary or contingent. (Definition)

5. Whatever is necessary has an essence identical to its existence. (Premise)

6. Whatever has an essence identical to its existence is Pure Actuality. (From 5)

7. Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause. (Premise, PSR)

8. Fundamental particles are necessary. (Assumption)

9. Therefore, fundamental particles are Pure Actuality. (From 6 and 7)

10. Fundamental particles are contingent. (Assumption)

11. Therefore, fundamental particles have an external cause. (From 7 and 10)

12. If fundamental particles have an external cause, that external cause is Pure Actuality. (Premise)

13. Therefore, Pure Actuality exists. (From 9 and 12)