Where is the True Church Found?

Martin Luther addresses this question not by looking for marks of success but by pointing to the seven marks of Jesus' presence.

“First, the holy Christian people are recognized by their possession of the holy word of God. ...wherever you hear or see this word preached, believed, professed, and lived, do not doubt that the true ecclesia sancta catholica, “a Christian holy people” must be there, even though their number is very small.

“Second, God’s people or the Christian holy people are recognized by the holy sacrament of baptism, wherever it is taught, believed, and administered correctly according to Christ’s ordinance.

“Third, God’s people, or Christian holy people, are recognized by the holy sacrament of the altar, wherever it is rightly administered, believed, and received, according to Christ’s institution.
...don’t be led astray by the question of whether the man who administers the sacrament is holy… Wherever you see this sacrament properly administered, there you may be assured of the presence of God’s people.

“Fourth, God’s people or holy Christians are recognized by the office of the keys exercised publicly. That is, as Christ decrees in Matthew 18 [:15–20], if a Christian sins, he should be reproved; and if he does not mend his ways, he should be bound in his sin and cast out. If he does mend his ways, he should be absolved.

“Fifth, the church is recognized externally by the fact that it consecrates or calls ministers... Wherever you see this done, be assured that God’s people, the holy Christian people, are present…

“Sixth, the holy Christian people are externally recognized by prayer, public praise, and thanksgiving to God. Where you see and hear the Lord’s Prayer prayed and taught; or psalms or other spiritual songs sung, in accordance with the word of God and the true faith; also the creed, the Ten Commandments, and the catechism used in public, you may rest assured that a holy Christian people of God are present.

“Seventh, the holy Christian people are externally recognized by the holy possession of the sacred cross. They must endure every misfortune and persecution, all kinds of trials and evil from the devil, the world, and the flesh (as the Lord’s Prayer indicates)...”

"On the Councils and the Church" 1539 in Luther's Works 41.