“For the holy and blessed Triad is indivisible and one in itself. When mention is made of the Father, there is included also his Word, and the Spirit who is in the Son. If the Son is named, the Father is in the Son, and the Spirit is not outside the Word. For there is from the Father one grace which is fulfilled through the Son in the Holy Spirit; and there is one divine nature, and one God ‘who is over all and through all and in all.”

St. Athanasius of Alexandria, Letters to Serapion on the Holy Spirit  (I.14)


See also:

Catholic Encyclopedia, The Blessed Trinity

Michael Barber, “Scripture and the Trinity: The Biblical Roots of the Dogma (Podcast for Trinity Sunday 2012)”

Nick Norelli, Trinitarian Resources