From St. Athanasius . . .
(St. Athanasius speaks of being made sons of God by the Spirit in Holy Baptism.)
Moreover, when He teaches us to pray, He says . . . 'When ye pray, say, Our Father, which art in heavens.' . . . He has bid us be baptized, not in the name of Unoriginate and Originate, not into the name of Uncreate and Creature, but into the name of Father, Son, and Holy Spirit, for with such an initiation we too are made sons verily . . . for it is because of the Son that the Father is so called by us; for since the Word bore our body and came to be in us, therefore by reason of the Word in us, is God called our Father. For the Spirit of the Word in us names through us His own Father as ours, which is the Apostle's meaning when he says, 'God hath sent forth the Spirit of His Son into your hearts, crying, Abba, Father.'
(from Defense of the Nicene Definition, §31
in Nicene and Post-Nicene Fathers, Vol, 4, p. 171-72)