Both "reality" and "illusion" are simply words. As words, they carry the definitions that their speakers intend for them to have, and that their listeners read into them. What this means is that "reality" and "illusion" both have multiple meanings, and their definitions in a particular circumstance must be determined by context.
For example, I could say that my brother is real, a part of reality. I could say that Tom Sawyer is not real. He is not part of reality. If I am speaking to my sister, she will understand and agree with me. We will have communicated effectively.
However, If Huckleberry Finn told Injun Joe that Tom Sawyer is not real, that he is an illusion, both my sister and I would agree that Huckleberry Finn is not speaking truthfully -- he is either wrong or lying.
Why is it true for me to say that Tom Sawyer is not real, but false for Huckleberry Finn to say the exact same thing?
The reason, like with tensed facts, is context.
In the context of the Tom Sawyer stories, Tom Sawyer is "real". In the context of our own lives, he is not.
Interestingly, much as Tom Sawyer is to Mark Twain, so we are to God.
Tom Sawyer is real within his own created context. We also are real within our own created context. Above that context, however, just like Tom Sawyer is to Mark Twain, we are made up. We are invented.
We are created.
So the difference between reality and illusion is often simply context. However, there is absolutely no coherent context in which it is true that God is illusion.