Which lens do I need when photographing a christening? Inside a church. I have a Nikon D3200.
Your Question is beyond broad in spectrum. You only stated what body you had and nothing about your level of skills or what lenses you already tested or looking to buy.
Lenses can range from 150$ (or less) second hand to 2000$ or more.
I will attempt to answer your question in detail since I own a D3200 myself.
I have the stock 18-55mm that comes with a Kit and I have a 35mm 1.8G prime lense that I shoot 99% of the time.
I have rented out a 24mm F1.4 and 17-55mm.
I also have Speedlight as my flash.
With that - the best combo are:
1) Any zoom lense with a F2.8 where you can get a focus shot on your subject if you cannot move into the subject
2) a 35mm prime or a 24mm (not the F1.4 as it is expensive) will enable you sharp images at a reasonable cost - but remember that a prime is a fix focal therefore no zoom, so make sure you are able to close the distance.
This is more than sufficient for you to get great picture AFTER you learn how to operate your D3200. Do not think that you can leave it on auto and it will do it for you.
The church is dim lit so anything at F2.8 will aid you for your need and then post processing will aid you to eliminate noise if you raise the ISO.
Again: any zoom lense F2.8 if you have to be stationary. 35mm or 50mm if you can position yourself.
Extra tip: Leave the flash at home unless you plan on taking pictures in the dark corner of the church
A prime lens, say 35mm or 50mm, are best for indoor photography where light intensity varies highly within small areas. They usually have a fast autofocus, good for events.
The 50mm lens is versatile enough to cope with group and portrait photos alike.
Within prime lenses, look for f1.2 to f2.0, they take the most light in.
In this layout, consider also an M42 universal adaptor for Nikon, so you can get very inexpensive M42 vintage manual focus lenses (e.g 10 $/EUR). For instance the Helios' 44m2 to 44m7 are well reputed. Especially the 44m7 has a very high resolution and brings up beautiful colours; the 44m2 is well known for its astonishing bokeh. Aside from these qualities, the manual focus requires good anticipation for snapshots.
If image quality is of utmost importance in portraying the event, consider the rental of a medium format digital camera and a set of lenses, e.g 35mm (group), 50mm and 85mm (portraits).
For group portrait photos, an option includes the use of panoramic photo software which stitches a sequence of images into a larger one.
A last comment, for Nikon D3200, lenses Nikon 35mm and Nikon 50mm may require their own autofocus motor. Worth checking this with the seller.