Well, I'm a protestant so there might be a different belief system in the Catholic church. "Grace by faith alone", which is the basis of what I commented above is a reformation doctrine and is mostly a protestant thing (some Catholics I have met believe it, but officially not the Roman Catholic Church), and penitence is entirely a Catholic thing, and was a primary cause of the reformation.
There are actually a LOT of different belief systems within Christianity and the related cults surrounding it, so you can't entirely sum them up by saying "Christianity". How I would respond to anyone stating that suffering is required to pay for sins is that it is un-Biblical and they've warped the message of Jesus, and are adding to/complicating the gospel.
This is possible in the Catholic church because edicts of the Pope and Councils have had equal (or at least near equal, I'm not a Catholic historian or theologian) weight with scripture, which of course have no weight at all to Protestantism as the Pope is just another person. Mother Theresa and penitents have such a drastically different belief system, that while I would not go so far as to call them non-Christian as I think only God alone determines that, I would say that their teachings are not the same religion as mine. Saints again are a purely Catholic thing, so to me and other protestants, Mother Theresa is just another person, and "Saint" is a worthless designation from the Catholic Church. In addition, individual adherents of what is classified as "Christian" can teach and believe (or fail in their belief) of the core doctrines and cease to become what I would consider Christian, despite doing things in the name of Christ. The church during the time of Paul's letters and Acts were dealing with the same thing, people using the name of Christ who were not what Luke or Paul considered Christians.
It sounds like a no-true-Scottsman argument, and perhaps it is, but the argument that I'm making is that you are lumping billions of people together, when #1 they don't lump themselves together (Catholics have considered protestants and the reformers heretics as well), and #2 they believe and practice drastically different things. You will likely hear Protestants hedge when asked if Roman Catholics are Christian, and the reason is because on an individual basis, there are Christians within the Roman Catholic church. But what they teach is not what we consider the Christian gospel.
