Nothing is must have but good gear is strongly recommended. The thing about weddings is that there is lots happening and there are pretty much no second chances. This means you have to work fast so you will rarely see wedding photographers with tons of gear unless they also have an assistant.

For Canon users, standard wedding gear would be a 5D Mark III, 24-70mm F/2.8L and 70-200mm F/2.8L. Compared to your Rebel T2i, these are rather expensive but they are so common that almost any place that rents photo gear will have these. Reserve as fast as you can and take them at least one day before the wedding so that you can practice. Gear that you are not familiar with is more dangerous than poor gear.

The other thing that is common is an external flash to light up your subjects and freeze the motion of people, particularly indoors. Since direct flash is rarely flattering, you need one that swivels to be bounced on walls and the ceiling. Check the wedding venue first. In some places, this is impossible to do. In that case, you should go with an off-camera flash either held by someone else or by a flash bracket that attaches to the bottom of your camera.