"The mystery of faith" and "a mystery of faith" are phrases found in different contexts and with a variety of meanings, either as translations of Greek τὸ μυστήριον τῆς πίστεως or Latin mysterium fidei or as independent English phrases.
The phrase "the mystery of faith" is given as a translation of the phrase "τὸ μυστήριον τῆς πίστεως" in 1 Timothy 3:9 in two English versions of the Bible: the Wycliffe Bible and the Douay-Rheims Bible. This translation of the text is exceptional, since far more numerous are the Bible versions that render the phrase as "the mystery of the faith", while others still have phrases like "the deep truths of the faith" (New International Version and New International Reader's Version); "the secret of the faith that God made known to us" (New Century Version); "the revealed truth of the faith" (Good News Translation); "the mystery of the Christian faith" (God's Word Translation); "the true faith that God has now made known to us" (Easy-to-Read Version); "what God has shown us about our faith" (Contemporary English Version); "the faith that has been revealed" (Common English Bible); "the secret of the faith" (Young's Literal Translation); "God's plan and what we believe" (Worldwide English).
The passage in question requires deacons to hold to this "mystery of faith", "the deep truths of the faith", "the true faith that God has now made known to us" or however else it can best be expressed. Richard C. H. Lenski identifies it with "the mystery of godliness" mentioned in verse 16 of the same chapter. According to Witness Lee, it is mainly Christ and the church. This mystery, Andrew Louth says, interrogates us rather than being questioned by us. Barnes' Notes on the Bible identifies the word "the faith" in this context with "the gospel", a view with which Gill's Exposition of the Entire Bible agrees, as does the People's New Testament, while Clarke's Commentary on the Bible remarks that one manuscripts gives, in place of "the faith", "the resurrection of the dead, which is one of the greatest mysteries of the faith. Floyd H. Barackman says that in this passage "the faith" refers to the whole of the New Testament.