Dear George Vergese,
Most of the Bible versions including the Catholic versions are ignored with the last part in Lord’s Prayer. However most of the researchers and scholars comments that, it was part of the Lord’s Prayer. The Peseto version of Bible which we follow also has this line added. Again, tt is worth reading the teaching of St. John Chrysostom on the Lord's Prayer. He mentions on Matthew 6:13 as-
"He again encourages and raises our spirits, 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."
...When He saith, "Thine is the kingdom," He sets before us even him, who is warring against us, brought into subjection, though he seem to oppose, God for a while permitting it. For in truth he too is among God's servants, though of the degraded class, and those guilty of offense; and he would not dare set upon any of his fellow servants, had he not first received license from above. And why say I, "his fellow servants?" Not even against swine did he venture any outrage, until He Himself allowed him (Lk 8:30-32); nor against flocks, nor herds, until he had received permission from above.
"And the power," saith He. Therefore, manifold as thy weakness may be, thou mayest of right be confident, having such a one to reign over thee, who is able fully to accomplish all, and that with ease, even by thee (Rom 10:2).
"And the glory, for ever. Amen." Thus He not only frees thee from the dangers that are approaching thee, but can make thee also glorious and illustrious. For as His power is great, so also is His glory unspeakable, and they are all boundless, and no end of them. Seest thou how He hath by every means anointed His Champion, and hath framed Him to be full of confidence?”
Hence I feel it is by its own taught by our Lord rather than a doxology added later.
>>Having reached this level which is furthest from purity, and in the manner that Christ Himself taught us, why do we add the doxology, not in the original Bible, which takes the Prayer back to its purer levels :
>>For Thine is the Kingdom, the Power and the Glory, for ever and ever.
In HIS Love,
Thomas Mann George KL,