A new edition has brought Walther’s Theses on Law and Gospel back onto the front burner of Lutheran discussion. One of the theses states essentially that it is wrong to deny that members of church bodies other than our own may be true members of the kingdom, since no association of men can save, but only that one which Christ gives through Holy Baptism.
When I was in seminary, Dr. Robert Kolb, described how Lutherans thought about other Christians in a metaphor that has always stuck with me as simple and helpful. The metaphor is certainly not his own, but the way he he applied it was helpful. The metaphor is scriptural: the church as a body. But for the sake of this teaching, he treated each church group as its own body.
Dr. Kolb said that any time we see a beating heart in a church body we rejoice and acknowledge the life of Christ therein. In other words, wherever it is believed that Jesus is the only way to salvation and that his work is the only work meritorious unto our eternal good, we rejoice in the work of the Spirit to create such faith through his word, and where administered, his sacraments.
But at the same time, if an arm is hanging by a thread or a great abscess is noticed, we cannot simply say, “Well, the heart is beating, who cares about anything else.” We realize that any ailment, can ultimately lead to death. And this is why we seek to dialogue with other Christians is regards to errors we see in their midst. While the beating heart is the sign of life, it is not necessarily the sign of health. And so we rejoice to have brothers and sisters in other churches, yet we do seek to correct ailments we see. We hope they will do the same for us if they believe us to not be well in regards to doctrine or related practice.
No doctor stops his examination once he has discovered a beating heart.  Neither should we.
ah yes, I have used this metaphor many times. It is very helpful.