You have actually picked one of the most difficult places to get a straight answer to that question.
The nearest church to Porttannachy would be the Enzie North church which is in the village of Portgordon. I do not know where the post-1855 registers of Enzie are.
However the Enzie is an enclave of Roman Catholicism, so Charles could have been baptised in the RC church. The nearest RC church would be one of the following: St Peter's in Buckie, St Ninian's at Tynet and St Gregory's at Preshome. I understand that the baptismal registers for all three are held at St Peter's.
The there is the possibility that your Petries were adherents of one of the Free churches. Again, I don't know where their registers are, if indeed they have survived.
Just to complicate matters, they might have been Episcopalian, in which case the baptism could have been in Gordon Chapel in Fochabers or in the Episcopal church in Buckie.
Finally, part of Enzie belongs to the parish of Rathven, and part to the parish of Bellie, so it is possible that the baptism was in either Bellie Parish Church, or in Rathven Parish Church which is on the other side of Buckie.
These days Buckie has a bewildering variety of Christian denominations. You will just need to hope that most of them arrived after 1880!