35 And if this stability seems to be open to exception, however rare the exception may be, as in the case of certain natural marriages between unbelievers, or amongst Christians in the case of those marriages which though valid have not been consummated, that exception does not depend on the will of men nor on that of any merely human power, but on divine law, of which the only guardian and interpreter is the Church of Christ. However, not even this power can ever affect for any cause whatsoever a Christian marriage which is valid and has been consummated, for as it is plain that here the marriage contract has its full completion, so, by the will of God, there is also the greatest firmness and indissolubility which may not be destroyed by any human authority.
Source: Casti Connubii (Vatican.va)