I have not studied Greek, and I wish I had. A knowledge of Greek is almost essential if you are going to be learned in theology. Of course if you are some sweaty goober Fundamentalist preacher bawling into your microphone in some auditorium masking as a church, you don't care.
This phrase of one substance was hotly debated at the Council of Nicaea in 325. The difference was between the terms homoousios and homoiousios. The word homoousios means "same substance", whereas the word homoiousious means "similar substance". Note the difference of the "i" or iota in Greek. That tiny difference was huge. The subtle Greek language was ideal for theology and this is why so many heresies and sects arose in the East.
The Council of Nicaea was convened, in large part to solve the dispute between the Arians and the orthodox. The Arians believed that Christ was a creation and not God. The orthodox believed in homoousios, that the Father and the Son, were of one substance, to wit: God. The homoiousios crowd, the ones who wanted to insert the "i" was the compromise position.
Some historians believe that the Emperor Constantine, who presided at the Council, threw his weight to the homoousios faction and put them over the top.
The Fathers of the Council had scruples about homoousios as it could not be directly found in the scriptures but had to be deduced from philosophy about the workings of the Trinity.
So, my children, if you believe in the Trinity, then you subscribe to homoousios and you can evidently thank the Emperor Constantine!
