The law regulating solemnisation of marriages among Indian Christians is laid down in the Indian Christian Marriage Act of 1872.
The operation of this Act is not confined to a marriage to which both the parties are Christians. A marriage to which one party alone is a Christian is also regulated by the provisions of this Act. In other words, this Act does not stand in the way of a Christian marrying a non-Christian, but such a marriage will have to be solemnised under the provisions of this Act. For a valid marriage under the Christian Marriage Act, two requirements should be satisfied, viz., (a) the marriage should be solemnised under the Act and (b) it should not offend the personal law of any of the parties to it. Therefore in a marriage where one party to it is a Christian or both parties to it are Christians, it should be solemnised under this Act and if not so solemnised, it would be void. In short, in a marriage under this Act, if one party thereto alone is a Christian, such a marriage becomes valid only if the personal law of the non-Christian Party treats such marriage as valid. Where the wife is a Christian woman and the husband is a Hindu, there is no prohibition under Hindu law for such a marriage.
Source: Wikipedia