While we do not discuss theology on here, I will venture to clarify on this for photography's sake.
Baptism comes in 2 basic flavors in practice. One is Adult Baptism in which people may be in street clothes or swim wear covered in a white or colored robe and fully immersed in an indoor pool or in a river. Others with a different understanding of Baptism will baptise babies, often in first 6 months or so. They are typically dressed in a bright white gown (even males) similar to a wedding dress. Surroundings are typically a bit dim in comparison, thus a high contrast situation.
I typically shoot +1/3 for most subjects, and for this child baptism I would go +1 EV. I might spot meter on the skin as an alternative... The important moments happen very fast, so being prepared is critical. That special moment of the water sprinkling on the babies forehead or the adult rising from the pool and the water running off the face and the smile it elicits, is fleeting. Try to do a pre shoot 1/2 an hour prior to the ceremony. You'll be prepared and more relaxed for the real thing.