"HERESY and SCHISM are distinguished in respect of those things to which each is opposed essentially and directly. For heresy is essentially opposed to faith, while schism is essentially opposed to the unity of ecclesiastical charity. Wherefore just as faith and charity are different virtues, although whoever lacks faith lacks charity, so too schism and heresy are different vices, although whoever is a heretic is also a schismatic, but not conversely. This is what Jerome says in his commentary on the Epistle to the Galatians [In Ep. ad Tit. iii, 10]: "I consider the difference between schism and heresy to be that heresy holds false doctrine while schism severs a man from the Church." Nevertheless, just as the loss of charity is the road to the loss of faith, according to 1 Timothy 1:6: "From which things," i.e. charity and the like, "some going astray, are turned aside into vain babbling," so too, schism is the road to heresy. Wherefore Jerome adds (In Ep. ad Tit. iii, 10) that "at the outset it is possible, in a certain respect, to find a difference between schism and heresy: yet there is no schism that does not devise some heresy for itself, that it may appear to have had a reason for separating from the Church.""
~St. Thomas Aquinas: Summa Theologiae, II-IIa, q. 39, art. 1, ad 3.
Read more: Q. 39