in Theology, a term of the same import with co-essential, denoting something of the same substance with another. The orthodox believe the Son of God to be consubstantial with the Father.

The term ἰσούσιος, consubstantial, was first adopted by the fathers of the councils of Antioch and Nice to express the orthodox doctrine more precisely, and to serve as a barrier and precaution against the errors and subtleties of the Arians, who owned everything except the consubstantiality. The Arians allowed that the Word was God, as having been made God, but they denied that he was the same God, and of the same substance with the Father; and accordingly they exerted themselves to the utmost to abolish the use of the word. The emperor Constantine used all his authority with the bishops to have it expunged out of the symbols; but it still maintained itself, and is at this day, as it was then, the distinguishing criterion between an Athanasian and an Arian.

Sandius and others maintain that the word consubstantial was unknown till the time of the council of Nice; but it is certain that it had been before proposed to the council of Antioch, in which Paul of Samosata had been condemned, though it had there the fortune to be rejected. Curcellæus, on the other hand, maintains that it was an innovation in doctrine in the council of Nice to admit an expression the use of which had been abolished by the council of Antioch.

According to Athanasius, the word consubstantial was only condemned in the council of Antioch inasmuch as it implied the idea of a pre-existent matter. Now, in this sense, it is certain the Father and the Son are not consubstantial, there having been no pre-existent matter.