"The Church exists by mission, just as a fire exists by burning."
- Emil Brunner
I am an outsider to all this; however . . . I can try. And I will offer . . . someone can tell me all they have to say about a philosopher or church leader's ideas. But I can't say what the person *really* means, unless I have seen how the person lives what that person claims is correct religion and philosophy and theology. And so, perhaps I can play with this stuff, any way I please, since I have not gotten to know any of these people personally . . . so I can know what their ideas mean, in their real lives.
Ok . . . so here goes > about the Church existing by mission > mission can have different meanings. It can mean going where the gospel has never been preached. It can mean sending missionaries to help newer church areas to be taught and discipled with correct beliefs and practices. It can mean to make sure the established Christians are cared for. But I most people can mean a mission is an outreach from a more established church area.
So, since I do not know exactly what Emil means . . . I am just offering what I have on this, in case this is somehow relevant. I may or may not agree with Emil.
I remember while I was living outside, on purpose as a missionary experiment. And I got to sharing with a certain church. They were totally receiving of me, though I was an outsider on the street. They were bountiful and hospitable and they had more people on Wednesday, than plenty of other churches did. And they had a missionary director, full-time, for a church with maybe only a few hundred people.
So, which came first, the chicken or the egg? I think the egg is the missionary, and you need a healthy and happy chicken to lay the egg!

By the way, they had that director, but I did not discover her until I had been with them . . . for some while. Missions was so helped, there, but it was not the centerpiece of it all, but I would say it was a well-supported by-product.
Yes, Jesus gave the early church the mission of reaching the world for Jesus. However, the church exists because of Jesus in us, and how Jesus also has had Peter feed His sheep. Jesus did not only send ones out, but He told Peter to feed His sheep. So, just reaching more and more people, as mission, is not enough. We need how to take care of the ones who get saved, plus the ones more and more maturing in Christ.
I suspect I have seen how ones can get people to get a quick-fix thing, and guarantee them they are going to Heaven. But they don't grow really and become sound in God's love with Jesus Christ's "rest for your souls" (in Matthew 11:29). And so, in case a ministry does not bring someone to trust in Jesus and submit to Him, it is possible God will not trust such a ministry with true conversions, since our Father expects us to feed and care for ones He trusts to us.
Often enough, to me it seems, the ones more active in the "mission field" can not do very well, possibly, unless there is the healthy group of sheep supporting them with prayer and money and other help. And so, the support people "at home" need to have healthy relating and growing, in Jesus. And it can be, how a missionary will be sure to come "home", at times, to personally feed the sheep, in order to help make sure they are getting well taken care of.
While the shepherd went out to find the lost sheep, someone was home tending to the flock!
Also, for local evangelism by an established church, we need to have healthy members so they can care for the newborns in Christ. And we have how we must have leaders who are "examples" for the other members and for the newborns >
"nor as being lords over those entrusted to you, but being examples to the flock." (1 Peter 5:3)
So > clearly, then, also, there must be departure from just proper beliefs and practices and sacraments. Ones need to become mature in Jesus, for living every moment, and not only appearing for pre-planned and canned local church activities. And there must be God's way of developing local families > after all, one of the basic requirements to qualify as a "bishop" is the person has learned how to rule his own house well > 1 Timothy 3:1-10. He knows, then, how to take care of God's people in our Father's family caring and sharing way > he did not learn this in seminary; seminary can help; but it is no substitute for maturing even for decades with his wife and children, much with her help to get real with God in Jesus Christ's way of loving so he becomes "blameless" the way God's love and correction ***cure*** a Christian's character to become blameless.
So . . . in case one's think mission means mainly to the lost, and it is the life and flame of Jesus Christ's church > I have heard ones say, perhaps like this, that evangelism is the main function of the church. But I offer how that could be like saying a family is mainly interested in adopting more and more children. And so, what happens to the ones already adopted???? That would not work right, at all.
In the New Testament I think I have seen more about what feeds and cares for the sheep, than about reaching new souls. Plus . . . when Jesus appointed His twelve > why did Jesus select His twelve? > in Mark 3:14 we have >
"that they might be with Him, and that He might send them out," we have in Mark 3:14.
He did not select them only for going out to preach and do other outreach things!
But Jesus wanted them to "be with Him". Our personal sharing with Jesus has us growing in Him and therefore in His love and His grace which we minister to one another, to feed ***us*** > so that "we", "speaking the truth in love, may grow up in all things into Him who is the Head---Christ---" > in Ephesians 4:14-15. After all, God's drive and flame was to have many children who are "conformed to the image of His Son, that He might be the firstborn among many brethren." (in Romans 8:29)
So, essential to this is first to have a person repent and become born-again . . . saved . . . yes, indeed. But now we have our process of growing in Jesus and maturing in how Jesus is and loves. And as we grow in Christ, now our cup runs over to bring others to Jesus ***and to sustain them*** and to help them also to grow in Christ.
But there is a "salvation only" sort of a thing . . . like to "faith only" and the "sola scripture" thingy. Ones can only or mainly be about reaching new souls. And then ones who got "saved" have an interesting way of not being behaved. And then comes the big discussion about if the person really got saved, or not. And then might come the idea that the person could even leave God and die but the person got saved, and so the person was unconditionally guaranteed to "go to Heaven". But ones have a way of not talking about first getting with Jesus, in this life, and growing in His grace. And ones kind of leave out the correction part > Hebrews 12:4-14 < which, I understand guarantees that any truly converted person is guaranteed to spend eternity with Jesus, yes, but part of this guarantee is how the truly converted person will get real correction by God so the person does not leave Him, plus grows in the perfection of God's love, as it is written >
"Love has been perfected among us in this: that we may have boldness in the day of judgment; because as He is, so are we in this world." (1 John 4:17)
So, what really has us ready for "the day of judgment" is how we have been getting conformed to the image of Jesus, in His perfect love curing our character so we are compatible with our Groom. Beliefs do not guarantee this, nor do correct practices and sacraments; God's real correction does > Hebrews 12:4-14.
And this is personal, of God in us, personally changing our character so we are like Jesus and therefore we relate in love like Jesus with our Father and with people. So much in scripture is about being perfected in Christ and how to relate as His family, though we do have plenty of God's word for unsaved people.
"Him we preach, warning every man and teaching every man in all wisdom, that we may present every man perfect in Christ Jesus. To this end I also labor, striving according to His working which works in me mightily." (Colossians 1:28-29)
I point out how Paul, here, says he ministers according to how God in him works Paul to minister. So, this is very personal, in sharing with God Himself ! ! ! And we also have >
"for it is God who works in you both to will and to do for His good pleasure." (Philippians 2:13)
So, indeed, then, Biblical Christianity means how God is transcendent but also so personal, working even in our willing and doing, so that our ministering is "according to His working" personally feeding and guiding us, every moment, then, I would consider. And God who is so transcendent is the One who knows how to do this, within us.
"Therefore submit to God." (in James 4:7)