4 Moreover, they use their knowledge of the historical fact that bishops of a former time have taught that Father and Son are of one substance, to subvert the truth by the ingenious plea that this is a heretical notion. They say that this term 'of one substance,' in the Greek homoousion, is used to mean and express that the Father is the same as the Son; that is, that He extended Himself out of infinity into the Virgin, and took a body from her, and gave to Himself, in the body which He had taken, the name of Son.
This is their first lie concerning the homoousion. Their next lie is that this word homoousion implies that Father and Son participate in something antecedent to Either and distinct from Both, and that a certain imaginary substance, or ousia, anterior to all matter whatsoever, has existed heretofore and been divided and wholly distributed between the Two; which proves, they say, that Each of the Two is of a nature pro-existent to Himself, and Each identical in matter with the Other.
And so they profess to condemn the confession of the homoousion on the ground that that term does not discriminate between Father and Son, and makes the Father subsequent in time to that matter which He has in common with the Son. And they have devised this third objection to the word homoousion, that its meaning, as they explain it, is that the Son derives His origin from a partition of the Father's substance, as though one object had been cut in two and He were the severed portion.
The meaning of 'one substance,' they say, is that the part cut off from the whole continues to share the nature of that from which it has been severed; but God, being impassible, cannot be divided, for, if He must submit to be lessened by division, He is subject to change, and will be rendered imperfect if His perfect substance leave Him to reside in the severed portion.
Source: On the Trinity (New Advent)