My recollection (from reading a variety of sources) is that the English heralds generally prefer not to grant a crest to a cleric, but will do so if requested. Usually this is reserved to later generations, but not necessarily.
And in support of Bill Henry, I would reaffirm that there is no substitute for research.
If you can prove a connection through descent from an armiger, there is a good chance that you can bear his arms, or a variation of them.
If you have done no research, or if your research has not demonstrated such a connection, you should look at a fresh design.
Regards,
Mike