Paul wrote three books that we call the "Pastoral Epistles" - I & II Timothy, and Titus. These are letters to ministers, which primarily talk about ministry - particularly, ministry in the local churches. They give us instruction on how people in the church should behave or conduct themselves in the worship and service of God. Paul tells us what should be done (or not done), how it should be done, and who should be doing it (I Timothy 3:15). I Timothy, chapter 3 gives instructions about the qualifications of bishops / overseers and deacons. As good leadership is established in the church, Paul wants everyone to remember how important the church's job is.

Consider I Timothy 3:14-16:

• Behave yourself in the house of God.
  Your conduct towards the church is important to God. See Revelation chapters 2 & 3 ("I know..."); Matthew 18:15-20; Ephesians 5:1-3; Philippians 2:14; 4:5; James 5:9; I Corinthians 14:26b, 40; 16:14.
• The house of the living God is the church.
  The church is not a physical building. It is a living relationship between God and the people of God. See Ephesians 2:19-22; I Peter 2:5.
• The church is the pillar and foundational ground of the truth.
  The church is responsible to support and to propagate the truth. As pillars hold up a roof and provide a shelter / pavilion, so the church provides a place where the truth can provide shelter for others. See Matthew 5:14-16 (cp. John 3:16-21); Matthew 28:18-20; Philippians 2:14-16; John 8:30-36; 18:37 cp. 20:21.
• The truth that the church upholds is the mystery of godliness.
  Godliness is "piety or reverence for the things of God." It is "the fear of God." To pursue the mystery of godliness is to ponder, "How can unbelieving, proud, selfish, sinful men become believing, humble, gracious lovers of God who now reverence God?" See Romans 1:1-7, 16-18; 3:19-26; 11:33-36; 16:25-27; I Corinthians 2:7-16, 4:1-2; Ephesians 1:3-9, 3:1-10. See also Psalm 130:3-4; I Timothy 1:12-16.
• The mystery of godliness is revealed through the knowledge of Christ.
  Christ is the key to all mysteries. As the knowledge of Christ is revealed, so then, is the mystery of Godliness revealed. See II Peter 1:2-4; Romans 8:28-39; I Corinthians 1:18-31; II Corinthians 4:1-6; Ephesians 5:25-32; Colossians 2:1-10.