My church building that is about a year and a half old has a white substance on the interior brick walls. What is it and what can be done about it?

It's caused by moisture. It's effervesence(spelling?). It can be removed with muratic acid (Dilute hydrochloric acid) which is available at a hardware store.

You have to find the source of the moisture.

Not sure how to seal brick, but it would need to be etched first with the acid.

Is it getting damp?
Is it powdery? Crystallized?

It could be mold, calcium build up,

You probably need to use a brick cleaner and then a brick sealer.
Inside the building you need something that isn't toxic to breath and do it in the warmer weather when you can open all doors and windows.

It is called efflorescence, caused by salts which are present in the wall and being pushed out by moisture. It may be from the morter drying up and will not come back once you have it cleaned, or there may be a source of water getting into the brick in which case it will continue to form. Have it cleaned and see if it comes back. Be careful with acids, they may burn or discolor the brick. Power washing with water or sandblasting can also be used. You should talk to whoever installed it for you before sealing it, make sure there is not a source of water or you may trap in inside the wall.

You are exactly right but I have found that using 25% diluted and doing several treatments saves your natural color.Be patient but it does work very well.I have a masonary background and starting a new business in concrete countertops so pass the word around and get an extraordinary product (comparible to any granite or marble tops without corner seams)for way less than half the cost.

It's efflorescence I agree. Be careful with acids, I don't think using Muriatic Acids should be used indoors unless the entire building is very well ventilated by openning all window and doors. The acid can ruin all metal items.

Read about efflorescence here. TCNA - Technical Services: FAQ

Jaz