If a person leaves in his will a substantial portion of his funds to the administration of a religion, can he still designate a member of that religion to be the executor of the estate?
Generally a person may designate any competent adult who is willing to be the executor. It is very common to designate a major beneficiary as executor. There is no rule against a possible conflict of interest, although an executor must treat all legatees fairly.
In many jurisdictions one can also designate a legal entity as an executor. When a will is complex adn involves large sums a bank or other financial institution may be designated as executor.