23 Let no one, then, be deceived by the distinction which some civil jurists have so strongly insisted upon - the distinction, namely, by virtue of which they sever the matrimonial contract from the sacrament, with intent to hand over the contract to the power and will of the rulers of the State, while reserving questions concerning the sacrament of the Church. A distinction, or rather severance, of this kind cannot be approved; for certain it is that in Christian marriage the contract is inseparable from the sacrament, and that, for this reason, the contract cannot be true and legitimate without being a sacrament as well. For Christ our Lord added to marriage the dignity of a sacrament; but marriage is the contract itself, whenever that contract is lawfully concluded.
Source: Arcanum Divinae (Vatican.va)