What is a "strong sender"? The first and most obvious way to measure this is numerical. We could count this either as (1) total number of missionaries sent or (2) number of missionaries per capita. (The difference between these two measures recalls the old story of the chicken and the pig going to breakfast: the chicken donates, but the pig is all in.) It's extremely difficult to figure out who the mission senders and receivers are, due to issues of security. I've recently touched on this issue here. That said, it's a bit easier to figure out how many are being sent out than to figure out where they are going. There are two groups that count missionaries: the Center for the Study of Global Christianity (CSGC) and Operation World (OW). CSGC counts all missionaries (Protestant, Catholic, Independent, etc), whereas OW counts "PIA" missionaries (Protestant, Independent, and Anglican). The CSGC numbers are about double the OW ones. Here's the CSGC numbers.
Higher “sent per million” can be interpreted as a strong signal of commitment on the part of the local church. However, this means little if the total number of workers sent isn’t enough to finish the task. The reality is, we need more workers. In terms of this need, a “strong sender” should really be defined as one that has a significant impact on the remaining task. What needs to happen to make a difference in this respect?
The 2018 Status of Global Mission (CSGC) estimates 2.1 billion people are unevangelized. This number is currently rising, heading toward 2.3 billion by 2025 (adding 200 million unevangelized people to the remaining task). To make a difference in this requires at least reversing this trend (enough so the task isn't growing), and at most, sending enough workers to finish the task entirely. Grappling with this requires at least three things:
Sufficient numbers. Let's suggest a country needs to send enough workers to tackle 1 percent of the remaining task: 1 percent of 2.3 billion is 23 million. I've argued a small team can impact 100,000 people. 23 million people broken into segments of 100,000 would require 230 teams. At roughly 2 to 3 people per team, this equals about 600 missionary workers. That doesn't seem like a lot, but long-term missionaries (especially those using a movement strategy) are not common. They are measured in terms of "workers per million believers." A strong sender would, therefore, likely have several million believers already.
Scalable strategy. A team of 2 or 3 people cannot evangelize 100,000 people (not to mention their future generations) on an additive (house by house by house) basis. For a small team to reach 100,000 people requires a movement strategy that can scale rapidly to the whole population. Teams that successfully implement a movement strategy most often come out of an existing movement: movements have the experience to start movements and multiply themselves. It is, therefore, likely that a strong sending nation would be one that has experienced movements or has movements happening in nearby countries, where experience can be gained.
Near-culture believers. Missiologists have long known near-culture evangelists are more effective than far-culture missionaries. (Translation and contextualization are huge investments by far-culture missionaries. Once accomplished, these enable near-culture evangelists to be far more effective.) The typical path of a movement is an outsider catalyzing near culture believers, who in turn are the main instruments in the work. So a strong sending country will likely be a nation that is culturally and geographically near to a pool of unevangelized individuals. (As an aside, for discussion in a future post: a lot of the "last mile" of evangelism will take the form of home mission, not foreign mission. If we are going to have a from-everywhere-to-everywhere approach we have to realize that foreign-vs-home-mission stats is an arbitrary division that is more confusing than necessary. The main question is not the political boundaries to be crossed, but rather the cultural and linguistic ones.)
In summary: "potential strong senders" are countries with several million believers, who have the capacity to send workers, whose believers have experienced movements, and who are near-culture to unevangelized populations.
So what’s the list of countries? Pulling a list of countries with more than 30 million believers, a "short" list of potential countries that seem to meet the requirements above would include: China, Ethiopia, India, Indonesia, Kenya and the United States.