After 539 Ravenna was reconquered by the Romans in the form of the Eastern Roman Empire (Byzantine Empire) and became the seat of the Exarchate of Ravenna. The greatest development of Christian mosaics unfolded in the second half of the 6th century. Outstanding examples of Byzantine mosaic art are the later phase mosaics in the Basilica of San Vitale and Basilica of Sant'Apollinare Nuovo. The mosaic depicting Emperor Saint Justinian I and Empress Theodora in the Basilica of San Vitale were executed shortly after the Byzantine conquest. The mosaics of the Basilica of Sant'Apollinare in Classe were made around 549. The anti-Arian theme is obvious in the apse mosaic of San Michele in Affricisco, executed in 545–547 (largely destroyed; the remains in Berlin).
Try to answer this question if possible (otherwise reply "unanswerable"): When did the biggest achievement in Christian mosaics take place?
the second half of the 6th century