If you have a suit I would wear it. This is a once in a lifetime event.
For the baby, not the father, we hope.
You should wear a suit, a solid colored shirt, a conservative tie and shined shoes.
I put that up under "depends". As in, depends on the church, your culture, etc.
I typically wear traditional clothes on my children's christenings.
You should wear your best church clothes. If you don't usually go to church, you should go at least once before the christening, and wear 1 step above the best-dressed congregant.
A day suit.