Question: Is the addition of the verse, "For Thine is the kingdom, the power and the glory" a Protestant addition to the Lord's Prayer, and if so, why was it adopted by the New Order?
Answer: The verse in question, sometimes called a doxology, is an addition to the scriptural form of the Lord's prayer, but it is many centuries older than Protestantism and the New Order. It can be found in Chapter 8 of the Didache, a patristic text going back almost to the time of the Apostles. It is not found in the King James version of the Bible in Luke xi, but is interpolated at the end of the prayer in Matthew vi. Very likely, it worked its way into an earlier text used by the translators, a copyist's error made by someone familiar with the version in the Didache.1
The same doxology has been part of the Catholic Byzantine Rite for centuries. The congregation recites or sings the Lord's Prayer (Otche Nash, in Church Slavonic) to which the priest responds with the doxology: "For Thine is the kingdom, power, and glory, of the Father, Son, and Holy Ghost, now and forever, world without end. The congregation responds "Amen."
The inclusion of the doxology in the Novus Ordo seems to be more an effort to curry favor with the Protestants than with the Orthodox who include the same mention of the Trinity as the Catholic Byzantines.