18 He who has seen the Son, has seen the Father: for in all things the Son is like to Him who begot Him; begotten Life of Life and Light of Light, Power of Power, God of God; and the characteristics of the Godhead are unchangeable in the Son; and he who is counted worthy to behold Godhead in the Son, attains to the fruition of the Father. This is not my word, but that of the Only-begotten: Have I been so long time with you, and have you not known Me, Philip?
He that has seen Me, has seen the Father</em>. And to be brief, let us neither separate them, nor make a confusion: neither say thou ever that the Son is foreign to the Father, nor admit those who say that the Father is at one time Father, and at another Son: for these are strange and impious statements, and not the doctrines of the Church. But the Father having begotten the Son, remained the Father and is not changed. He begot Wisdom, yet lost not wisdom Himself; and begot Power, yet became not weak: He begot God, but lost not His own Godhead: and neither did He lose anything Himself by diminution or change; nor has He who was begotten any thing wanting.
Perfect is He who begot, Perfect that which was begotten: God was He who begot, God He who was begotten; God of all Himself, yet entitling the Father His own God. For He is not ashamed to say, I ascend unto My Father and your Father, and to My God and your God.
Source: Catechetical Lectures (New Advent)