I have a John P. Pentecost b 1852 to Robert T. Pentecost and Mary Catherine Fields of Carroll Co., MS. He is still with his parents as of the 1880 census. I have no record of him after that. Wonder if he might be father to your John P. Pentecost. I should add that Mary Catherine's father was Pearson Fields.
Have you found your John P. in any census records? in the WWI draft cards? I'd like to help you work backwards to see if there is a connection.