You’ve undoubtedly heard this one: if each believer won one person to Christ each year (or month, or whatever) within a short time the whole world would be saved.
This approach never works because of the limits of our social circles. We presume that everyone in the world is connected to everyone else.
In fact, each of us typically has a few ‘strong’ connections (close family and friends), and some ‘weak’ connections (to colleagues and others). Strong connections provide support, encouragement, etc. Weak connections often lead us to opportunities that we otherwise wouldn’t hear about (because you’re strong connections are like you, and often know most of the kinds of things you know).
Some people have a lot of ‘weak’ connections, and they become ‘hubs.’ Information often passes through them—thus they are natural gateways, and can stop the flow of information if they desire. While it is theoretically true that everyone in the world is connected (e.g. “six degrees of separation”), in fact the non Christian world is cut off from casual relationships with much of the Christian world. Getting “over” the hub gateways into other social circles takes a lot of effort.
In the “each-one-win-one”, Christians would rapidly run out of people to win if they stay within their existing social networks—typically, their strong connections, weak connections, and perhaps “second order” (friend of friend) connections.
We are more comfortable with the idea of winning friends and family, and less comfortable with the idea of seeking out complete strangers. It is these disconnects–when people avoid other types of people–that cause the break down of world evangelization. Intentionally intersecting a new social circle is hard work but work that must be done. Mission is about leaving one’s existing social circle, and intersecting a new one.