What does it mean to be a faithful church? What is a true and pure church in the eyes of God? What does it mean to be sound, healthy, and vibrant?
Maybe it is a spiritual church. A mature, spirit filled church has the "mind of Christ (1 Cor 2:16)." This is not about precise doctrinal formulas, but about behavior. A quick reading of the immediate context and of the entire letter to the Corinthians demonstrates that this is about godly behavior.
"And I, brethren, could not speak to you as to spiritual men, but as to men of flesh, as to infants in Christ. … you are still fleshly. For since there is jealousy and strife among you, are you not fleshly…?"(1 Cor 3:1-3)
The immaturity of the Corinthians was reflected in their behavior. This is why Paul spends nearly an entire chapter dealing with the excellence of love (1 Cor 13:1f), which is superior to both faith and hope. A spiritual church is a loving church.
Maybe it is a church that teaches sound doctrine. Once again, this is not about precise, hair splitting doctrinal formulations.
"If anyone advocates a different doctrine and does not agree with sound words, those of our Lord Jesus Christ, and with the doctrine conforming to godliness, he is conceited and understands nothing; but he has a morbid interest in controversial questions and disputes about words, out of which arise envy, strife, abusive language, evil suspicions, and constant friction between men of depraved mind and deprived of the truth…" (1 Tim 6:3-5).
Sound doctrine and godliness connects with sound behavior and attitudes. Disputes over words and hair splitting doctrinal formulations usually stifles the missional emphasis of the church. Perhaps this is why reformation and early enlightenment churches were not overtly missional.
"But as for you, speak the things which are fitting for sound doctrine. Older men are to be temperate, dignified, sensible, sound in faith, in love, in perseverance. Older women likewise are to be reverent in their behavior, not malicious gossips nor enslaved to much wine, teaching what is good, so that they may encourage the young women to love their husbands, to love their children, to be sensible, pure, workers at home, kind, being subject to their own husbands, so that the word of God will not be dishonored. Likewise urge the young men to be sensible; in all things show yourself to be an example of good deeds, with purity in doctrine, dignified, sound in speech which is beyond reproach, so that the opponent will be put to shame, having nothing bad to say about us" (Tit 2:1-8).
Sound doctrine has every bit as much to do with how you say what you say and how you behave as it does with what you say. Godly behavior ensures that the "word of God will not be dishonored."
This means that all sound teaching stems from the greatest commands, loving God and loving your neighbor (Mt 22:37-40). Love is the greatest thing. Love is what give credibility to our witness.
To sum it up, the sound, faithful, vibrant church is the one that truly takes these greatest commands to heart and demonstrates it in their behavior.