Homoousious (literally, “same substance”) is a term that became very important in church history and was at the center of a controversy in the early fourth century leading up the Council of Nicea.
Homoousious is a compound Greek word. The word homo simply means “same.” Most people would recognize this from the word homosexual. The term same sex is often used in place of homosexual as in “same-sex marriage.” Heterosexual starts with the Greek word hetero, which means “different.”
Ousious is the Greek word for “substance or being.” It refers to the essential nature of something—what it really is. Homoousious would be “of the same substance.” Heteroousious would be “of a different substance.” There is one more word that is important in this context: homoiousious. The extra i in homoiousious changes the meaning to “of a similar substance.”
So, in Greek, in the early fourth century, if two things were described as homoousious, it would mean that they were exactly the same kind of thing. If they were described as heteroousious, it would mean that they were different kinds of things. And if they were described as homoiousious, it would mean that they were similar—neither was exactly like the other, but not so different as to be described as hetero.
It was in the early fourth century that a man named Arius began teaching that Jesus was homoiousious with the Father—of a “similar substance.” According to Arianism, Jesus was “divine” but not in exactly the same way the Father was divine. He did not teach that Jesus was heteroousious, which would have indicated that Jesus was simply human or radically different from the Father in some other way; rather, he taught that Jesus was “similar” to the Father. In response, a bishop named Athanasius insisted that Jesus was homoousious with the Father—of the “same substance”—the same kind of being. In other words, Jesus is divine in the same way as the Father is.
The issue was settled at the Council of Nicaea. The bishops of the church discussed and debated the issue and finally decided by overwhelming majority that homoousious (“same substance”) best encapsulates the teaching of the New Testament on the nature of Jesus. The Nicene Creed was the result of this meeting and reads in part, “We believe in one Lord, Jesus Christ, the only Son of God, eternally begotten of the Father, God from God, Light from Light, true God from true God, begotten, not made, of one Being with the Father.” That last phrase of one being is a translation of the Greek word homoousious. The word homoousious is not found in Scripture, but the Council felt this was the best word to describe orthodox biblical teaching concerning Christ.
We find the teaching that the Father and the Son are homoousious in John 1:1, Philippians 2:6, and Colossians 2:9. More importantly, the concept of homoousious explains the entire sweep of the New Testament teaching about the Person and nature of Jesus Christ. Jesus is fully God in the same way that the Father is God—they are of the same divine nature.