Two of the most confusing words in Christian theology, homoousius means "of the same substance/nature" while homoiosius means "of similar substance/nature." The importance of the two terms grew out of the Arian controversy of the 4th Cent. Arius had taught that Jesus was simply human, a heresy challenged by the trinitarian party led by Athanasius, who insisted that Jesus was God and of the "same" nature, i.e., homoousius, with the Father. In the heat of the controversy some party suggested a compromise solution with the formula that the Son was homoiosius, i.e., 'similar' to the Father. Arius was prepared to go along with this, but the trinitarians realized that such a declaration would be spell the death of Christian orthodoxy and refused. Both the Council of Nicea (325 AD), as well as the Council of Constantinople (381) affirmed that the Son was homoousius, of the same nature with the Father.
See also Trinity
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