There is a fundamental difference between a marriage that has been declared null and a marriage that is dissolved. The former states that while a marriage was presumed to be valid, it was adjudicated by Church courts that the presumption was erroneous, for it was never a valid marriage. On the other hand, the latter upholds the validity of a marriage that is now being dissolved under certain conditions.
A sacramental marriage, between two baptized persons, Catholic or not, becomes absolutely indissoluble when the union is consummated by the spouses through sexual intercourse performed in a human manner. Canon 1141 dictates that no power on earth is capable of dissolving such a sacramental and consummated union.
However, the Church acknowledges that there are degrees of stability in a marriage, based on the relationship between baptism and the marital consent of the spouses. Thus, the consummated marriage between a baptized and a non-baptized person, and the marriage between two non-baptized persons, although they may be valid, are not indissoluble because they are not sacramental. The valid but non-consummated marriage between two baptized persons can also be dissolved, since it is not indissoluble. The rationale for this is that although the Gospels establish the indissolubility of marriage, it is left to the competent Church authority to determine the degree of stability in a valid marriage. The competent Church authority is vested in the Roman Pontiff and those he delegates to act in his name in such petitions. When it is the case of a valid union between two non-baptized persons, the converting party can exercise his or her right to dissolve the union, and the competent Church authority verifies the existence of the right.
For the difference between the Pauline Privilege and the Privilege of the Favor of the Faith (Petrine Privilege), please click here.