Catalyzes the condensation of formaldehyde with tetrahydromethanopterin (H(4)MPT) to 5,10-methylenetetrahydromethanopterin. 5,6,7,8-tetrahydromethanopterin + formaldehyde = 5,10-methylenetetrahydromethanopterin + H2O Belongs to the formaldehyde-activating enzyme family.