4 As to their blasphemous position that “the Son knows not the Father perfectly,” we ought not to wonder at it; for having once set themselves to fight against Christ, they contradict even His express words, since He says, “As the Father knows Me, even so know I the Father.” Now if the Father knows the Son but in part, then it is evident that the Son does not know the Father perfectly; but if it is not lawful to say this, but the Father does know the Son perfectly, then it is evident that as the Father knows His own Word, so also the Word knows His own Father Whose Word He is.
Source: Deposition of Arius (New Advent)