A reading from the First Letters to Serapion by St. Athanasius
If God is Trinity, which He actually is, and a Trinity which has been shown to be indivisible and not dissimilar to itself, then its Holiness must be One, and its Eternity One, and its immutable nature.
The faith in the Trinity, which has been handed down to us, is One, and it is this faith which unites us to God. After all, it is a consecration in the Trinity! The one who divides the Son from the Father or demotes the Spirit to the level of creatures has neither the Son nor the Father but is godless and worse than an unbeliever (1 Tim 5:8) and anything but a Christian. And this is just and right, for as the baptism given in the Father and the Son and the Holy Spirit is One and the faith in the Trinity is One, as the apostle says: “One Lord, one faith, one baptism” (Eph 4:5), so the Holy Trinity, which is identical with itself and united with itself, has nothing of what is originated within it. This is the indivisible unity of the Trinity and faith in this Trinity is One.
I have expressed what I have learned, which is in conformity with the Holy Scriptures and is consistent with the passages from the Holy Scriptures. It is not an invention brought in from the outside, but the Lord Jesus Christ Himself personally taught the Samaritan woman—and through her, He teaches us—the perfection of the Holy Trinity as One inseparable Godhead. The Truth Himself gives witness when He says to the Samaritan woman: “Believe me, woman, the hour is coming and is now here when true worshippers will worship the Father in Spirit and Truth. And indeed, the Father seeks such people to worship Him. God is Spirit, and those who worship him must worship in Spirit and Truth” (Jn 4:21, 23–24). It is clear from this passage that Truth is the Son Himself, for He says, “I am the Truth” (Jn 14:6). It is He who was invoked by the prophet David when he said, “Send out your light and your truth” (Ps 43: 3). Therefore, the true worshippers indeed worship the Father, but in Spirit and Truth, confessing the Son and in Him, the Spirit. For the Spirit is inseparable from the Son, as the Son is inseparable from the Father. The Truth Himself gives witness when He says, “I will send you the Paraclete, the Spirit of Truth, who proceeds from the Father (Jn 15:26), whom the world cannot receive (Jn 14: 17)”— that is to say, those who deny that the Spirit is from the Father and in the Son. It is necessary, therefore, to confess and cling to the Truth, according to the model of true worshippers. As for those who, after all this, are still unwilling to learn and incapable of understanding, let them at least cease from foul speech and not separate the Trinity, lest they be separated from life.
Book Reference:
Anatolios, Khaled. Athanasius. The Early Church Fathers. New York: Routledge, 2004.