Athanasius ad Serapion 1:28 & 33

28. But, beyond these sayings, let us look at the very tradition, teaching, and faith of the Catholic Church from the beginning, which the Lord gave, the Apostles preached, and the Fathers kept. Upon this the Church is founded, and he who should fall away from it would not be a Christian, and should no longer be so called. There is, then, a Triad, holy and complete, confessed to be God in Father, Son, and Holy Spirit, having nothing foreign or external mixed with it, not composed of one that creates and one that is originated, but all creative; and it is consistent and in nature indivisible, and its activity is one. The Father does all things through the Word in the Holy Spirit. Thus the unity of the holy Triad is preserved. Thus one God is preached in the Church, ‘who is over all, and through all, and in all’ — ‘over all’, as Father, as beginning, as fountain; ‘through all’, through the Word; ‘in all’, in the Holy Spirit. It is a Triad not only in name and form of speech, but in truth and actuality. For as the Father is he that is, so also his Word is one that is and God over all. And the Holy Spirit is not without actual existence, but exists and has true being. Less than these (Persons) the Catholic Church does not hold, lest she sink to the level of the modern Jews, imitators of Caiaphas, and to the level of Sabellius. Nor does she add to them by speculation, lest she be carried into the polytheism of the heathen. And that they may know this to be the faith of the Church, let them learn how the Lord, when sending forth the Apostles, ordered them to lay this foundation for the Church, saying: ‘Go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit.’ The Apostles went, and thus they taught; and this is the preaching that extends to the whole Church which is under heaven.