John Calvin Commentary 1 Timothy 3:9

"holding the mystery of the faith in a pure conscience." — 1 Timothy 3:9 (ASV)

Holding the mystery of faith. As if he had said: “Holding the pure doctrine of religion from the heart, with a sincere fear of God”; or, “Being well instructed in the faith, so as not to be ignorant of anything Christians need to know.” He gives to the sum of Christian doctrine the honorable title of a mystery; for indeed God, through the gospel, reveals to people on earth a wisdom that angels in heaven behold with admiration, and therefore, we need not wonder if it exceeds human capacity.

Let us therefore remember that it should be embraced with the deepest reverence. Because we could never, by our own strength, ascend to such a height, let us humbly entreat God to impart it to us by the Spirit of revelation. On the other hand, when we see wicked people either ridicule these doctrines or have no relish for them, let us acknowledge that it is due to the grace of God that those things that have been hidden from others are in our hearts and before our eyes, as Moses says (Deuteronomy 30:11).

Thus, he wishes that deacons should be well instructed in the mystery of faith; for although they do not hold the office of teaching, it would still be exceedingly absurd for them to hold a public office in the Church while being poorly informed in the Christian faith, especially since they will frequently need to administer advice and consolation if they are not to neglect their duties. It is added, in a pure conscience, which extends to the whole life, but chiefly so that they may know how to obey God.