What would be the best way to market traditional retail space to churches/religious users?

First question if you are the landlord. If you have retail center:
What is the impact of a church to the rest of your center?
Take into accunt their service hours, days, evening etc?
Parking situation.
Wear and tear on the leased premises.
What type of renovations are customary for a church.
Size of leased spaced.
Occupancy limits.
Most churches for the right price they will find you.

I would start by calling the local denominational offices to find out what churches might be looking for facilities and for a list of recent church plants. I would also do direct marketing to existing churches, primarily those already leasing non-traditional worship space.

If available, contact the broker of any existing vacant church within 50 miles. Chances are every "looking" church or worship group in the area will have contacted the broker for an inspection.
Also start-up churches, Pentacostal and non-denominational churches will often locate in non-traditional locations. Call the local churches and just tell them you have the space available and they may know a young pastor who is looking.
Contact seminaries in your state and see if you can place an advertisement with them. New graduates often found new churches and they need meeting places.