Jamieson, Fausset and Brown on 1 Timothy 3:9

9. the mystery of the faith--holding the faith, which to the natural man remains a mystery, but which has been revealed by the Spirit to them (Ro 16:25; 1Co 2:7-10), in a pure conscience (1Ti 1:5, 19). ("Pure," that is, in which nothing base or foreign is intermixed [Tittmann]). Though deacons were not ordinarily called on to preach (Stephen and Philip are not exceptions to this, since it was as evangelists, rather than as deacons, they preached), yet as being office-bearers in the Church, and having much intercourse with all the members, they especially needed to have this characteristic, which every Christian ought to have.