We believe in the Communion of Saints. We do not stand for a gathering of like-minded individuals. We do not stand for a club for "spiritual people".
We stand for the people of God called by the Holy Spirit and gathered around His Word and Sacraments.
More importantly, however, Jesus Christ stands for the Church. He is its foundation, its head, its bridegroom. He is the King and the Church stands or falls by Him.
In other Words, since Jesus lives forever, the Church shall endure forever. This is not because of the hard work of its members, but because of the grace of God showered in His Word.
Where is the Church? It is wherever the Gospel, the good news, is preached in its truth and purity.
Who is the Church? Those who have faith in Jesus Christ as their Savior from sin, death, and hell.
You can always find the Church, but you cannot always know who is in it because only God sees faith in hearts.
Why is this so important? You really cannot improve upon the words of the Small Catechism: "In this Christian Church He [the Holy Spirit] daily and richly forgives all my sins and the sins of all believers."
[Tomorrow: We stand for Baptism!]