You may be thinking of "Christ is amongst us (Khristos posredi nas)" to which the reply is "He is and shall be (Yest i byst)". This is used most often when the faithful approach to venerate the cross held by the priest, at anointing, etc. (outside the Paschal and Nativity seasons, of course).
What is used during the Nativity season? I've looked everywhere I know to for it and haven't been able to find it. Unless you mean, "Christ is born!" "Glorify Him!" for the Christmas season, which I know.