Because it promises to be a bride or groom, husband or wife, for life means a covenant with God. Need an oath to him The guests attending the ceremony as witnesses.
Because of that promise will be the bride or groom is a life-long marriage. to refer to the Covenant with God. The Declaration must have his guests to participate in the ceremony as a witness.
He said because of the commitment to the bride and groom as a husband or wife for life means that there is a covenant that God had to say to his vow to him, There is a visitor who came to participate in the ceremony to witness