Ignatius Antioch

The mystery of the Trinity consists in this, that God being numerically and individually one, exists in three Persons, or, in other words, that the divine essence, which is one and the same in the strictest and most absolute sense, exists in three Persons really distinct from each other, and yet each really identical with the same divine essence.  The Father is unbegotten, the Son begotten, the Holy Spirit proceeds from the Father and the Son as from one principle.  

Each Person is really distinct from the other, each is the true, eternal God, and yet there is only one God.  We can understand how three individual men are distinct from each other and yet possess humanity in common.  The unity of the three divine Persons is altogether different.  When we speak of them as one God, we mean not only that each is God, but that each is one and the same God.  This is the mystery of the Trinity, incomprehensible to any created intelligence.