St. John Chrysostom on schism:
"Nothing so provokes God to anger as the division of the Church." (PG, Vol. LXII, col. 85)
"The most pernicious of all things is to split the Church." (PG, Vol. LXI, col. 11)
"To cause a schism in the Church is not a lesser evil than to fall into heresy."
(PG Vol. LXII, col. 87)
"Not even the blood of martyrdom avails to wipe out this sin." (PG, Vol. LXII, col. 85)
St. Nicodemus the Hagiorite on schism:
Divine Chrysostom says (in his eleventh homily on the Epistle to the Ephesians)that a certain saint said that not even the blood of martyrdom can wipe out the sin of separating the Church and dividing it; and that for one to split the Church (i.e., create a schism) is a worse evil than that of falling into a heresy.
Dionysios of Alexandria the confessor wrote in his epistle to Bishop Nauatus that one ought to suffer any evil whatever rather than split the Church; and that the martyrdom is more glorious which one would have to undergo in order to avoid splitting the Church than the martyrdom which one would have to undergo in order to avoid becoming an idolater, since in the case of martyrdom to avoid becoming an idolater one becomes a martyr for the benefit of his own soul, whereas in martyrdom to avoid splitting the Church one becomes a martyr for the benefit and union of the whole Church.