2 But what shall we do? Are there, then, two beginnings? Let us beware of saying so. What then, if both the Father is the beginning and the Son the beginning, how are there not two beginnings? In the same way that we call the Father God, and the Son God, and yet say not that there are two Gods; and yet He who is the Father is not the Son, He who is the Son is not the Father; and the Holy Spirit, the Spirit of the Father and of the Son, is neither the Father nor the Son. Although, then, as Catholic ears have been taught in the bosom of mother Church, neither He who is the Father is the Son, nor He who is the Son is the Father, nor is the Holy Spirit, of the Father and of the Son, either the Son or the Father, yet we say not that there are three Gods; although, if we are asked of each apart, we must, of whichever we are questioned, confess that He is God.
Source: Tractates on the Gospel of John (New Advent)