Finally, in the light of later usage and the order of instruction give in the Didache, it appears probable that, immediately before or after baptism, the Lord’s Prayer was imparted to the catechumen, a conclusion that receives corroboration from such passages as Gal. 4, 6 and Rom. 8, 15. At all events, the passages referred to lead to the assumption that Paul desires to remind the readers of some well known prayer, offered by them at their common services and beginning with “Father”. Thus also the juxtaposition of the Aramaic “Abba” and the Greek ὁ πάτἡρ is most satisfactorily explained in the following manner: Just as the Christians of Greek speech used the Aramaic words Amen, Hallelujah, Hosannah, Maranatha unchanged in their worship, so likewise the Aramaic “Abba”, both to designate the Lord’s Prayer and to begin it, adding, moreover, the Greek equivalent. All the facts bearing upon the matter favor this assumption: Having become children of God through baptism, and having experienced the impartation of the Holy Spirit, they then offered the prayer of the children of God.
J. Michael Reu
Catechetics p 20