What is the one of the main differences between the Old and the new calendar Orthodox Church?

Dear Friends one of the many difference is that the old calendar Orthodox Church keeps undefiled the Orthodox faith and cannot be with those (new calendar church) who let modernism to undermined the true faith. It is very important to remind to all our readers the miracle that happened in 1925 on the eve, of the feast of the Exaltation of the All-Honourable and Life-giving Cross of our Saviour, 26th- 27th September (according to the Old calendar Orthodox Church) , the all-night vigil was served at the church of St. John the Theologian in suburban Athens. By 9 o'clock that evening, more than 2,000 of the true-Orthodox faithful had gathered in and around the church for the service. At 11:30 P.M., there began to appear in the heavens above the church, in the direction of North-East, a bright, radiant Cross of light. The light not only illuminated the church and the faithful but, in its rays, the stars of the clear, cloudless sky became dim and the church-yard was filled with an almost tangible light. The form of the Cross itself was an especially dense light and it could be clearly seen as a Byzantine cross with an angular cross bar toward the bottom. This heavenly miracle lasted for half an hour, until midnight, and then the Cross began slowly to raise up vertically. So its very important to follow the correct calendar and not the Gregorian. There are plenty of spiritual issues why we don't agree with the new calendar Orthodox Church.