In my "wandering years" I struggled a great deal with the issue of "salvation outside of the church." I define church as anyone who believes in Jesus (whether they are active in church or not), which is, the body of believers. I remember reading Justin Martyr (103-165AD) who was one of the earliest Apologists for Christianity and was beheaded for the faith. I recall the sense of thrill in reading about the Logos (Word) which created the world and permeates the world. The Logos/Word became flesh in Jesus, but Justin indicated that the Logos was at work in all of creation.
Like any early church philosopher/theologian, some of Justin's thought is inaccurate. Some of what he taught would be considered in error based on later councils. So I make no claims of inerrancy or infallibility here, but I do think it helpful to know that faithful Christians (dying for the faith seems to be a fruit of that claim) have struggled with the same questions which we face today. So I conclude with the reaffirmation that there is no salvation outside of Jesus Christ the Lord and I have every reason to hope that Jesus will find a way to gather into His kingdom as many people as possible. How does He do it? I am not sure. What I am sure about is the typical Christian spends precious little time and very little resources in reaching the lost for Christ. The typical Christian really is not consumed with the billions of people who do not know or trust Jesus. I also know that in most places and in most times, the behaviors of Christians (especially ministers) has driven large number of people away. The flaws of human beings are very apparent inside the church. If we are God's only hope to reach the world then God is in a tough spot.
I think the idea of Kingdom conveys a different slant on this. In Kingdom theology, there is a real sense in which God is absent. Jesus calls Satan the "prince of this world." There is a great deal of absence/presence imagery in the teaching of Jesus. There is a sense of our vocation being a time of preparation and waiting. The feel of the NT is God is coming here, to us, not that we are going there, to heaven. Death is not the passage to new life, resurrection is. "I saw coming down from heaven, a New Jerusalem!"
I believe the kingdom ethic (holiness, justice, righteousness) is a vital part of that preparation. If our focus is less on "going to heaven" (escape from here) and more about "bringing heaven to earth" then our ministries and evangelism have a different look. Our politics matters more, as does our stewardship and relationships, when this world is considered part of God's redemptive plan.
When I focus on going to heaven, I focus on me. I have to, I do not control anyone else.
When I focus on Kingdom, it is a 'we' thing. Suddenly our relationships matter because we are in it together.
When I focus on God the coming King, I am freed up from some of the nagging questions the agnostics or atheists throw out: "If God is good why is there so much evil?" I can say, "God is good and He is coming to save us. In fact, the rescue mission began long ago and reached its climax in the life, death and resurrection of Jesus. He is the KING. He is the ONE in whom all our hopes are secure. Now we must, today, live as He lived, in faithful service to God and one another."
If some people enter the Kingdom after they die does that negate our work now? I think not. We are awaiting God's final deliverance, but our efforts now are vital to Him and of use to Him. So proclaiming Jesus and living as His people makes all the sense in the world. The world matters, so we preach the 'Jesus Way' to make the world more livable. The more people embrace Jesus and come to faith, the more our world will be kingdomlike and more livable. We cannot create paradise, only God can do that, but we can worship and serve God here, much better than we do now. And it is better to include all the world's inhabitants in that enterprise.
In the end, I think we must choose which quote from Jesus is our motto: "He who is not with Me is against Me" or "He who is not against us is with us." There is exegesis required to interpret each of these verses, and in a sense they refer to different things. But most of the time they end up being a motto for us and express how we see the world. Either option can, to the extreme, create problems. Held in tension they can provide us with a gracious approach to other humans even as we welcome them into relationship with Jesus and life as His possession.
We pray "Come Lord Jesus" because we need deliverance here and now. All of us, Jew, Christian, other believer or non-believer. The gates are open and the invitations have been sent out. Many, sadly, will choose to reject the King and refuse to come to the wedding banquet. I hope most will come, even if some come late. I pray for God to have mercy on us all. And I hope to live like one who knows He is a guest by grace!