Saint Athanasius the Great Quote (On the Holy Trinity)

We acknowledge the Trinity, holy and perfect, to consist of the Father, the Son and the Holy Spirit. In this Trinity there is no intrusion of any alien element or of anything from outside, nor is the Trinity a blend of creative and created being. It is a wholly creative and energizing reality, self-consistent and undivided in its active power, for the Father makes all things through the Word and in the Holy Spirit, and in this way the unity of the holy Trinity is preserved. Accordingly, in the Church, one God is preached, one God who is above all things and through all things and in all things. God is above all things as Father, for he is principle and source; he is through all things through the Word; and he is in all things in the Holy Spirit.

– Saint Athanasius the Great from a letter

Saint Quote: Saint Gregory of Neocaesarea (On the Holy Trinity)

There is one God, the Father of the living Word, who is His subsistent Wisdom and Power and Eternal Image: perfect Begetter of the perfect Begotten, Father of the only-begotten Son. 

There is one Lord, Only of the Only, God of God, Image and Likeness of Deity, Efficient Word, Wisdom comprehensive of the constitution of all things, and Power formative of the whole creation, true Son of true Father, Invisible of Invisible, and Incorruptible of Incorruptible, and Immortal of Immortal and Eternal of Eternal. 

And there is One Holy Spirit, having His subsistence from God, and being made manifest by the Son, to wit to men: Image of the Son, Perfect Image of the Perfect; Life, the Cause of the living; Holy Fount; Sanctity, the Supplier, or Leader, of Sanctification; in whom is manifested God the Father, who is above all and in all, and God the Son, who is through all. 

There is a perfect Trinity, in glory and eternity and sovereignty, neither divided nor estranged. Wherefore there is nothing either created or in servitude in the Trinity; nor anything superinduced, as if at some former period it was non-existent, and at some later period it was introduced. And thus neither was the Son ever wanting to the Father, nor the Spirit to the Son; but without variation and without change, the same Trinity abideth ever. 

-- Saint Gregory of Neocaesarea

Saint Basil the Great

The Godhead is common; the fatherhood particular. We must therefore combine the two and say, 'I believe in God the Father.'

The like course must be pursued in the confession of the Son; we must combine the particular with the common and say 'I believe in God the Son,' so in the case of the Holy Ghost we must make our utterance conform to the appellation and say 'in God the Holy Ghost.'

Hence it results that there is a satisfactory preservation of the unity by the confession of the one Godhead, while in the distinction of the individual properties regarded in each there is the confession of the peculiar properties of the Persons.

-- Saint Basil the Great