Grammar Check

"Him who is and was and who is to come"—

There is a Greek problem with the grammar here, because the present tense of the be verb doesn't actually agree with the subject. It indicates that a reference to Exodus 3, where God revealed himself as the “I Am” is in mind. The fact that God is also the God who was and who is to be indicates that all ages are ruled by God—He has never ceased to rule.

A better translation might be: "He who am and who was and who is to come." That would at least correctly communicate the grammar difficulty here.

Revelation has a number of grammatical "errors". Some say they are because of the Semitic origin of the writer, thus Greek being his second language. I, and some others, think that the "errors" are on purpose-- pointing back to the Hebrew Bible.