The Gloria Patri is a short expression of praise to the Trinity from the very early Church. Authors such as Hippolytus (@ 235) and Origen (@ 231) use very similar phrases as they are generally considered the great early thinkers/worshippers who articulated the notion of the Trinity. The form of the prayer became fixed to what we have today by the Council of Nicaea in 325 AD. Interestingly enough, this doxology was the Christian response to the “Arianism heresy.”
Arianism was first proposed early in the 4th century by the Alexandrian presbyter Arius. He suggested that Christ was not truly divine but a created being. Arius’ basic premise was the uniqueness of God, who is alone self-existent and immutable; the Son, who is not self-existent, cannot be God.
“According to the bishop Athanasius, Arius’ teaching reduced the Son to a demigod, reintroduced polytheism (since worship of the Son was not abandoned), and undermined the Christian concept of redemption since only he who was truly God could be deemed to have reconciled man to the Godhead.” (Encyclopedia Britannica).
The Nicene Creed emerged to clearly state that the Son is homoousion tō Patri (“of one substance with the Father”), thus declaring Him to be all that the Father is; and not only He, but He with the person of the Holy Spirit, make up the mystery that is the Trinity: God the Father, God the Son, and God the Holy Spirit.
You may be interested in hearing and seeing Bach’s Magnificat being performed with the Latin form of this prayer:
GLORIA Patri, et Filio, et Spiritui Sancto. Sicut erat in principio, et nunc, et semper, et in saecula saeculorum. Amen.
Simply, the Gloria Patri is in a long line of one sentence prayers meant to be as much meditative (mantra-like) as to be a statement of orthodox truth about God – who is by nature, Triune and Eternal:
GLORY be to the Father, and to the Son, and to the Holy Spirit. As it was in the beginning, is now, and will be forever. Amen.