The following is an excerpt from Dr. Thomas Holland's Crowned With Glory, ©2000, used with permission.
Matthew 6:13 - "For thine is the kingdom, and the power, and the glory, for ever."
And lead us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever. Amen.
The passage in question is the conclusion of what is commonly known as The Lord's Prayer. The prayer ends with the doxology, "For thine is the kingdom, and the power, and the glory, for ever. Amen." This phrase is found in the majority of Greek manuscripts, the Greek Textus Receptus and Majority Text, and is the reading of early English versions, the KJV, and the NKJV. It is not found in the main body of the Critical Text or most modern versions.
Some have argued that the prayer is the same as the one found in Luke 11:2-4. In that passage the doxology does not appear. It is then suggested that scribes who had a habit of harmonizing various passages in the four Gospels did so with this prayer.  While the two passages are similar in content it is doubtful they are the same prayer. The passage in Matthew is given for the multitude when Jesus preached His celebrated Sermon on the Mount. The passage in Luke is given specifically for the disciples of the Lord when asked how they should pray. Similarity, it should be remembered, does not mean sameness. Nor is it a surprise to find this prayer, or at least a form of it, appearing on more than one occasion.
The question then arises: "Did the prayer in Matthew originally contain the concluding phrase as found in the Traditional Text?" Among the Greek uncials it is found in W (fifth century), L (eighth century), 0233 (eighth century), K (ninth century), D (ninth century), Q (ninth century), and P (tenth century). It is found in the majority of all Greek minuscules such as: 28, 33, 565, 700, 892, 1009, 1010, 1071, 1079, 1195, 1216, 1230, 1241, 1242, 1365, 1546, 1646, 2174 (dating from the ninth century to the twelfth century). It is also found in the majority of all existing Greek lectionaries. Therefore, the weight of the Greek witnesses argues for its inclusion and validity.
It is likewise found in several ancient translations such as some Old Latin manuscripts, the Old Syrian, and some Coptic versions. The Syriac Peshitta (second/third century  ) reads, "And bring us not into temptation, but deliver us from evil: For thine is the kingdom, and the power, and the glory, for ever and ever: Amen."  Therefore, the reading embraces antiquity as well as geographical support.
The passage also has patristic support. The distinguished orthodox father of the fourth century, John Chrysostom, cites this passage. He writes, "by bringing to our remembrance the King under whom we are arrayed, and signifying him to be more powerful than all. 'For thine,' saith he, 'is the kingdom, and the power, and the glory'."  The oldest witness, which outdates all Greek manuscripts containing Matthew chapter six, is the Didache (otherwise known as the Teaching of the Twelve Apostles). This ancient catechism dates to the early second century, shortly after 100 AD, and contains a form of The Lord's Prayer:
But let not your fasts be with the hypocrites; for they fast on the second and fifth day of the week; but do ye fast on the fourth day and the Preparation (Friday). Neither pray as the hypocrites; but as the Lord commanded in His Gospel, thus pray: Our Father who art in heaven, hallowed be Thy name. Thy kingdom come. Thy will be done, as in heaven, so on earth. Give us today our daily (needful) bread, and forgive us our debt as we also forgive our debtors. And bring us not into temptation, but deliver us from evil; for Thine is the power and the glory for ever. Thrice in the day thus pray. 
Finally, in his studies on old papyri, Dr. George Milligan includes a sixth century prayer that incorporates the prayer of Matthew 6:13. Despite the fact that this papyrus is badly worn, it clearly contains the phrase in question.  The textual evidence for the traditional reading is both ancient and massive, and should be retained in our English translations.
 Bruce M. Metzger, The Text Of The New Testament (third ed., Oxford: The Clarendon Press, 1973), 197.
 Textual scholar F. C. Burkitt argued that the Peshitta did not exist before the fifth century. Although most scholars no longer hold to Burkitt's theory, most date the Peshitta to this time period. Others, such as E. Hills and A. Voobus have placed the origin of the Peshitta in the second century.
 James Murdock, The Syriac New Testament (Boston: H. L. Hastings, 1896), 9.
 St. Chrysostom, "Homily XIX," The Preaching of Chrysostom (ed. Joroslav Pelikan, Philadelphia: Fortress Press), 145.
 Teaching of the Twelve Apostles, 8:1
 George Milligan, Sections From The Greek Papyri (Cambridge: University Press, 1912), 132-134. Interestingly, Dr. Milligan notes of this papyri that it contains, "a passage which some may be tempted to quote in support of the A.V. rendering of Mt. VI.13." Since the phrase is included in this personal prayer, Dr. Milligan is correct in both his understanding of the origin of the quote and in its support for the Authorized Version.