Catalyzes the reversible interconversion of 5-formyl-H(4)MPT to methenyl-H(4)MPT(+). 5,10-methenyl-5,6,7,8-tetrahydromethanopterin + H2O = H(+) + N(5)-formyl-5,6,7,8-tetrahydromethanopterin One-carbon metabolism; methanogenesis from CO(2); 5,10-methenyl-5,6,7,8-tetrahydromethanopterin from CO(2): step 3/3. Belongs to the MCH family.