Complex sentence: In the midst of his literary labours Mesrop revisited the districts he had evangelized in his earlier years, and, after the death of Isaac in 440, looked after the spiritual administration of the patriarchate.
Split into simpler sentences: In the midst of his literary labours Mesrop did not neglect the spiritual needs of the people. He revisited the districts he had evangelized in his earlier years, and, after the death of Isaac in 440, looked after the spiritual administration of the patriarchate.