If your ratio is 1 : 50, then for every 1 ft of the model, you have 50 feet of actual church. Thus, if the model is 3.5 ft, then
1 / 50 = 3.5 / x
where x is the actual height of the church. Cross multiply and solve for x.
1 * x = 50 * 3.5
x = 175
Thus, the actual church is 175 ft