What is a true church?

Difference between a true church and a healthy church?
esse v. bene esse
true v. false
healthy v. unhealthy
regular v. irregular

Calvin, from 1 Corinthians 1:2,
What ground, then, had Paul for recognizing a Church at Corinth? It was this: that he saw among them the doctrine of the gospel, baptism, the Lord’s Supper — tokens by which a Church ought to be judged of. For although some had begun to have doubts as to the resurrection, the error not having spread over the entire body, the name of the Church and its reality are not thereby affected. Some faults had crept in among them in the administration of the Supper, discipline and propriety of conduct had very much declined: despising the simplicity of the gospel, they had given themselves up to show and pomp; and in consequence of the ambition of their ministers, they were split into various parties. Notwithstanding of this, however, inasmuch as they retained fundamental doctrine: as the one God was adored among them, and was invoked in the name of Christ: as they placed their dependence for salvation upon Christ, and, had a ministry not altogether corrupted: there was, on these accounts, a Church still existing among them. Accordingly, wherever the worship of God is preserved uninfringed, and that fundamental doctrine, of which I have spoken, remains, we must without hesitation conclude that in that case a Church exists.