Bill Hybels and Mark Mittelberg in their book, Becoming a Contagious Christian, provide a great formula for evangelism:
Close Proximity + High Potency + Clear Communication = Maximum Impact
Let’s briefly unpack this formula. In order to effectively evangelize today, you usually need to be in close proximity to someone in order to have the most impact. I find this true in the U.S. and in Colombia. Sure, people may listen to you if you share the gospel on the street, but I’ve seen that usually God uses a relationship in order to bring someone to Christ. How many non-Christians do you regularly meet with in order to build a relationship?
“High potency” in their book speaks about living a life of personal holiness and integrity. If you have friendships with non-Christians but are just like they are, you will not have any impact for Christ. You can even preach the gospel, but if you don’t live it, you’re just another hypocrite.
“Clear communication” of the gospel message is also vital. Many Christians think they can “live” the gospel without speaking it. Nope, that doesn’t work. We are called to preach/share/testify about Christ and about his gospel. Christ died for sinners, was buried, and rose again. The only way to be saved is to believe in him.
Now, Hybels and Mittelberg may have wanted to include prayer in “High Potency” (my copy is in Colombia so I can’t check on that), but I think the formula would be even better if we included another element, giving us this final formula:
Close Proximity + High Potency + Clear Communication + Focused Prayer = Maximum Impact
If we are not praying for growth in evangelism, for help to live holy lives, for wisdom to share the gospel clearly, and for specific unsaved people to be saved, I doubt very highly we will see Maximum Impact. Let me know if you have seen maximum impact without prayer, but I have not.
Now, this formula would be a great reference point for praying for missionaries. For example, based on Close Proximity you could pray, “Father, help our missionary build a new relationship today.” “Give them wisdom to find new contacts. Give them wisdom to know how to draw nearer to the contacts they already have,” and so on. I won’t parse that out any further, but I think the idea is clear.