It should be clear that I am not a pantheist, but a classical theist. Just three arguments in favor of classical theism I support are the First Way, the Fifth Way, and the Argument from Desire. Nevertheless, I'd like to defend pantheism for the sake of argument.
1. I exist. (Premise, contingent a priori)
2. (Hence) Some--i.e. least one--thing exists. (From 1)
3. Whenever some things exist, there is some thing of which they are all parts. (Premise, from mereology)
4. (Hence) There is exactly one thing of which every thing is a part. (From 2, 3)
5. The unique thing of which every thing is a part is God. (Definition, pantheism)
6. (Hence) God exists. (From 4, 5)
The argument is logically valid, but it is unsound. The defeater is found in the arguments of classical theism.