Religion on a broad scale probably helped us trust and work with one another outside of our initial family, tribal and city groups.

In the smallest social circles, you knew to trust your family and your tribe.
As they grew, you knew to trust your city and perhaps your nation-state.
At some point, society became complex and warranted the need to trust merchants and travelers from other countries and far away lands. This became easier if you knew the guy was catholic/christian/etc. You could then assume a common set of moral values.

Now though, its a crutch we shouldn't need. People are people, most of them are good and trustworthy.