It’s not so straightforward.
What makes a Christian a Catholic is the fact that his is in communion with other Catholics. Communion is a relationship which involves shared belief, but more than just shared belief – it also involves shared celebration of the sacraments, particularly the eucharist, shared worship, a commitment to a shared pursuit of the gospel, etc.
Given the various dimensions of this relationship, it’s not a simple binary that you are either in this relationship with other Catholics, or not. There can be various defects or weaknesses in the relationship, but it requires something fairly major to sever it entirely.
There isn’t a categorical statement of the propositions of faith that, if rejected, will completely destroy communion. The closest that there is to this is probably the Nicene Creed, which I think emerged from a statement of the faith-propositions which candidates for baptism were required by the early church to affirm. Reject that, and it’s pretty well impossible to be a Catholic. On the other hand, you can accept the Nicene Creed in a sense, or with an understanding, which differs from the sense or understanding which other Catholics may have. And, more to the point, there are many Christians who accept the entirety of the Nicene Creed, and yet are not Catholics, because for other reasons they are not in communion (and, mostly, don’t want to be).
So, while faith is an important element of communion, communion can’t be reduced simply to shared faith.
As to repentance, all Christians believe that everyone is called to repentance. But repentance, too, is not a simple binary, such that you either repent immediately and completely or not at all. Repentance is a change of heart leading to a change of life; it’s a process. If the Catholic church consisted only of people who had wholly and perfectly repented, it would be a much smaller church.
Ceasing to be a Catholic is somewhat like ceasing to be an American. You can be a bad citizen of the US, you can be angry at the US, or at America, you can take on Frenchified airs, profess revolutionary socialism, or learn how to make a good cup of tea. You can renounce your nationality. You can do things that arouse the ire and suspicion of other Americans. But none of this will necessarily mean that you stop being American.
Really, the only certain way to not be a Catholic is to want to not be a Catholic; to sever intentionally the relationship of communion that binds you to other Catholics.