In Revelation chapters 2 & 3, Jesus tells various churches in Asia Minor that they are spiritually healthy and others they are unhealthy. A man can be rich with a large estate and many servants yet physically be in poor health. Similarly, a church may have a beautiful building with a large congregation, yet spiritually be in poor health. Jesus makes clear to those churches in Asia Minor that the spiritually healthy ones would live but the unhealthy ones would die if they didn't repent.
a) Just as you need nutritious food for good physical health, the church needs to be fed with the word of God for spiritual health.
b) Just as you need your lungs working well so you can breathe properly for good physical health. Similarly, if a church is going to be spiritually healthy, the members must give themselves to individual and collective prayer. Prayer is the vital breath of the church.
c) Just as you need an atmosphere that isn't polluted, clean air for good physical health. A church to be spiritually healthy must avoid being polluted by the sinful ways, ideas and practices of the ungodly world.
d) Jut as you need exercise for good health, so the church needs to individually witness to Christ and spread the gospel locally and throughout the world to be spiritually healthy.
A healthy church will have a high proportion of its number at the prayer meetings. A high proportion of the income of a spiritually healthy church will be given to foreign missions. No matter how spiritually healthy a church is, it still needs to keep close to Christ to remain healthy. When a church is unwell, the answer is not for you to find fault but to rather be sincerely praying to God,
"O Holy Ghost revival comes from Thee, send a revival and start the work in me!"