Hi
Church records in Canton Berne usually start around mid 16th century for baptisms and marriages and late 16th century for burial records.
For Belp, the church records of baptisms and marriages start in 1549:
http://www.query.sta.be.ch/detail.aspx?id=220534
I can't find earlier burial records than for 1728, but maybe they are included in other books or they are missing.
To reach years before the church records, other documents are needed, such as contracts, testaments, documents about distributions of an estate or any other documents that might be available. Such documents might be found in the State Archive of Canton Berne. But it's not said that there ever existed any such documents about this family, that they exist now and if they still exist, they made their way to an archive. Some documents at State Archive of Canton Berne are online in their catalogue (see link above) and can be searched through full text. But not all their documents are already in their online catalogue, as much as I know. Also, they don't have time to do look ups.
Other possible archive might be the Belp municipality archive, but those usually are small and don't have an online catalogue.
So for best chances, you might want to hire a local professional genealogist. Adresses can be found at the Genealogische Heraldische Gesellschaft Bern under "Professionelle Genealogen":
https://www.ghgb.ch/forschung-links.html or at State Archive of Canton Berne under "Spezialistinnen und Spezialisten"
https://www.staatsarchiv.sta.be.ch/de/start/fuer-private/familienforschung.html
I hope this helps a little.
