It is an evil which prevails everywhere among mankind, that every one sets himself above others
…and especially that those who seem in anything to excel cannot well endure their inferiors to be on an equality with themselves. And then there is so much [a gloomy disposition] almost in all, that individuals would gladly make churches for themselves if they could; for they find it so difficult to accommodate themselves to the ways and habits of others. …
Extremely needed, therefore, by us all is the admonition to be stimulated to love and not to envy, and not to separate from those whom God has joined to us, but to embrace with brotherly kindness all those who are united to us in faith.
And surely it behoves us the more earnestly to cultivate unity, as the more eagerly watchful Satan is, either to tear us by any means from the Church, or stealthily to seduce us from it.
And such would be the happy effect, were no one to please himself too much, and were all of us to preserve this one object, mutually to provoke one another to love, and to allow no emulation among ourselves, but that of doing good works.
For doubtless the contempt of the brethren, [gloominess], envy, immoderate estimate of ourselves, and other sinful impulses, clearly shew that our love is either very cold, or does not at all exist.
Having said, “Not forsaking the assembling together,” he adds, But exhorting one another; by which he intimates that all the godly ought by all means possible to exert themselves in the work of gathering together the Church on every side; for we are called by the Lord on this condition, that every one should afterwards strive to lead others to the truth, to restore the wandering to the right way, to extend a helping hand to the fallen, to win over those who are without. But if we ought to bestow so much labour on those who are yet aliens to the flock of Christ, how much more diligence is required in exhorting the brethren whom God has already joined to us?
—John Calvin, 1509-1564
Commentary on the Epistle of Paul the Apostle to the Hebrews, 240–241.