They are to be grave, probably right thinkers. Double tongued meaning to be truthful in all they say, especially not to gossip or anything like that.
I must say that individual Christians should be everything that deacons are supposed to be. We are all in need of revival.
Paul is writing to tell Timothy how to behave himself as part of the church. He calls it the church of the "Living God" the pillar and ground of the truth.
Now we have the term; the mystery of godliness. This to me is an interesting term. Why did Paul say it that way? Godliness is what we all need to be, but I believe we need a pattern. The pattern is Christ. He is God manifest in the flesh. Justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory. That is my pattern for me to live a godly life. Romans 8:29 says, "We have been predestined to be conformed to the image of God's Son."