You ask some important questions. More in depth responses are available thru Catholic Answers. I will try to be brief here.
Sin (personal) is a knowing and deliberate violation of God's law. For it to be mortal the "matter" must be serious, for example one of the Ten Commandments. Deliberately missing Mass would be a mortal sin (assuming that the person realized the serious obligation, cf. Dies Domini). A non-Catholic would presumably not understand that obligation. If he did, of course, his salvation would depend on taking the step to joining the Church. Baptism would wash away all his sins. If he were baptized in another Christian Church, he would have to receive the sacrament of confession to obtain forgiveness of sins committed after baptism. Vatican II taught the necessity of membership in the Catholic Church to be saved. The traditional teaching still stands: "Outside the Catholic Church there is no salvation." This clearly flows from a positive response to the three basic questions. I hasten to add we can never judge the state of any other person's soul, e.g., whether their not being Catholic is somehow their own fault. However, the question we can ask: Is it ours? At a minimum have we prayed for that other person's conversion?
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