St Gregory of Sinai -- On the Trinity

The Trinity is simple unity, unqualified and uncompounded.

It is three-in-one, for God is three-personed, each person wholly interpenetrating the others without any loss of distinct personal identity.

God reveals and manifests Himself in all things in a threefold manner.

In Himself He is undetermined; but through the Son in the Holy Spirit He sustains and watches over all things. And wherever He expresses Himself, none of the three Persons is manifest or to be perceived apart from or without the other two.

St Gregory of Sinai