Complex sentence: In the late 14th and in the 15th century the Church Slavonic religious writings in East Slavonic territories underwent an archaization (rebulgarisation) to counteract the "falsification" of the divine word allegedly caused by the influence of vernaculars.
Split into simpler sentences: In the late 14th and in the 15th century the Church Slavonic religious writings in East Slavonic territories underwent an archaization known as rebulgarisation. The purpose of this archaization was to counteract the "falsification" of the divine word which allegedly had been caused by the influence of vernaculars.