For a Catholic, schism is always a sin. We may never separate ourselves from the Body of Christ. (Protestants who appeal to the dispute between St. Paul and St. Barnabas as a justification for schism are, from the Catholic point of view, falsely interpreting that account, mistaking a difference in mission as a difference in communion.) To separate ourselves from the Body of Christ is to separate ourselves from Christ. When the Church excommunicates a heretic, the Church is not committing schism, or placing herself in schism. The person being excommunicated, by his rebellion and obstinacy, is separating himself from the Church. Excommunication formally acknowledges this separation and implements the changes in sacramental practice viz-a-viz that person that must accompany his sinful actions.
In the opinion of some non-Catholics, however, being in schism is justified when the Church has fallen into heresy. Notice that this non-Catholic claim about the occasion when schism is justified assumes (contrary to the teaching of the fathers) that there is no See that is both (1) protected from heresy in its ex cathedra teachings and (2) is the rock upon which the Church is built, i.e. is that See with which we must be in communion in order not to be in schism. So, for example, according to this non-Catholic opinion, if I think the rest of the Church has fallen into heresy, then, ceteris paribus, I do no wrong by cutting myself off from the rest of the Church. But why then call it schism? Why not simply say in that case: "I and all who presently agree with me are the Church; those in heresy are not the Church; schism is always wrong, and I am not in schism since I and all who presently agree with me are the entirety of the Church."
This non-Catholic opinion therefore has a problem. It simultaneously treats heresy as insufficient to place its proponents outside of the Church (for we do not say that we are in schism with pagans, and yet this non-Catholic position claims that we are justified in making a schism from heretics), and yet sufficient to justify placing oneself out of communion with them. If heresy places those who hold it outside the Church, then it is impossible to be in schism from heretics unless we are also in schism from pagans. But if heresy does not place its holders outside the Church, then we are not justified in making a schism from them, for we are not justified in dividing the Body of Christ.
The Catholic position does not have this problem because we do not claim to be in schism from anyone, nor do we claim that schism can ever be justified. From the Catholic point of view, where there are schisms, they are schisms from (i.e. separations from, not separations by) the Holy See, or from those in communion with the Holy See. (see CCC #2089) This follows from St. Peter's role as the Church's principium unitatis.
I raise this issue because I believe that we cannot understand true unity without understanding what schism is. And we cannot attain true unity if we do not understand true unity. That is why I believe that we need to understand what schism is in order to attain true unity.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)