Peter Papuotsis, who is a Greek, is actually translating the LXX right now for the Holy Orthodox Church. He said this,
I know this Greek as Ecclesiastical Greek which is derived from koine Greek. However, I would not call it simplified. This is polished and refined Greek, although still konie. By the time the Creeds were written, they were written by educated Greek-Speakers so its more of a refined konie Greek, but not a simplied version. Simplified konie is found in some part of the New testament.
However, if you were to compare the Creed Greek with St. Paul's Greek you would find that they are the same quality.
I hope this helps. It has been a mystery to me for about 2 weeks. I have been memorizing it so I can pray the Rosary in Greek, and the Apostles Creed is at the beginning of the Rosary. I have the Lords Prayer down pat and also the Hail Mary in Greek. I truely wish I had some people I know who were studying Greek or were Greek speakers, it would really help me learn quicker. I have attempted to learn to speak Greek but I usually lose interest after awhile because I have no one to converse with. Unlike Spanish, where just about everybody knows some of it, Greek is one of those languages hardly anybody in Kentucky knows it or even thinks about it. I have been learning to read it, and have been studying it for about a year and a half. I can read it fairly good for what time I have been learning it, but I feel totally lost if I try to speak it. Oh well, I guess this is for another thread for another time. haha