Can we get married in a catholic church without his christening certificate?
Your fiance needs to put in a reasonable search for his baptismal certificate. He can ask his parents, godparents, older siblings, grandparents, aunts, uncles — maybe one or more of them could remember what church he was baptized in. Or, if he knew where his family was living at the time of his christening, he could contact all the churches within that area, give them his birthdate and see if they can find the certificate. The priest should be able to offer some more ways to find the certificate. Also, you also need to present yours because they want the documentation of your sacrament as well. If you can’t find any certificates at all, I imagine the priest will suggest that you be conditionally baptized (a baptism performed just in case you weren’t baptized properly before now).
