It was probably added by a copyist that used to hear this doxology in the celebration of the Eucharist just after the Our Father. Today you can still find it in the same place in the Divine Liturgy of St. John Chrysostom
, where it is said by the priest after the people have recited the Our Father:
á½Ï„Î¹ ÏƒÎ¿á¿¦ á¼ÏƒÏ„Î¹Î½ á¼¡ Î²Î±ÏƒÎ¹Î»ÎµÎ¯Î± ÎºÎ±á½¶ á¼¡ Î´ÏÎ½Î±Î¼Î¹Ï‚ ÎºÎ±á½¶ á¼¡ Î´ÏŒÎ¾Î±, Ï„Î¿á¿¦ Î Î±Ï„Ïá½¸Ï‚ ÎºÎ±á½¶ Ï„Î¿á¿¦ Î¥á¼±Î¿á¿¦ ÎºÎ±á½¶ Ï„Î¿á¿¦ á¼‰Î³Î¯Î¿Ï… Î Î½ÎµÏÎ¼Î±Ï„Î¿Ï‚, Î½á¿¦Î½ ÎºÎ±á½¶ á¼€Îµá½¶ ÎºÎ±á½¶ Îµá¼°Ï‚ Ï„Î¿á½ºÏ‚ Î±á¼°á¿¶Î½Î±Ï‚ Ï„á¿¶Î½ Î±á¼°ÏŽÎ½Ï‰Î½.
As you can see, later a reference to the Trinity was also added.