The bishop and the elder are the same, having moral authority in the assembly. Since the day of the apostles there are no appointed elders, but the spiritual mind discerns those who are elders. They should know how to rule their own houses first; they should not be novices, having a good report from those in the world. (Since the deacon is to be the husband of one wife, it is supposed that this be true of the elder, also.)
Deacons were to be grave, not given to wine or the love of money, and they were to hold the faith in a pure conscience. They must first be proved.
The deacon cared more for the practical needs of the saints while the elder was concerned more for their spiritual needs. For both, their own houses should be in order and their children should be subject. Their wives must be faithful.
Paul, when writing, hoped to be with Timothy in a short time. In case he delayed, he gave instruction and teaching about the testimony of the house of God. The house of God is the Church of the living God, the pillar and ground of the truth. No other ground of truth is found in this world.
Paul instructed Timothy in the greatness of the mystery of godliness. God came down to earth in flesh as a man. He manifested perfect holiness in His walk through this world. Angels saw Him as a man in this world; they will worship Him when He comes again (Heb. 1:6). Now He is hid in God (Col. 3:2). Christ was preached unto the Gentiles. This was new. Before God's testimony was only to Israel.
Christ was the first man received up into heaven in the presence of God. This gives us who are one with Him our place there. Think how great a company will fill the Father's house, yea, the courts of heaven in that approaching day, because the Son of God was believed on in this world.