3 For no one can know the Father, unless through the Word of God, that is, unless by the Son revealing [Him]; neither can he have knowledge of the Son, unless through the good pleasure of the Father. But the Son performs the good pleasure of the Father; for the Father sends, and the Son is sent, and comes. And His Word knows that His Father is, as far as regards us, invisible and infinite; and since He cannot be declared [by any one else], He does Himself declare Him to us; and, on the other hand, it is the Father alone who knows His own Word.
And both these truths has our Lord declared. Wherefore the Son reveals the knowledge of the Father through His own manifestation. For the manifestation of the Son is the knowledge of the Father; for all things are manifested through the Word. In order, therefore, that we might know that the Son who came is He who imparts to those believing on Him a knowledge of the Father, He said to His disciples: “No man knows the Son but the Father, nor the Father but the Son, and those to whomsoever the Son shall reveal Him;” thus setting Himself forth and the Father as He [really] is, that we may not receive any other Father, except Him who is revealed by the Son.
Source: Adversus haereses (New Advent)