The “mysteries” are the sacrament of baptism, and the “three-fold question” those which preceded baptism, viz.: Dost thou believe in God the Father Almighty? Dost thou believe in our Lord Jesus Christ, and in His cross? and Dost thou believe in the Holy Spirit? with the answer, “I believe,” to each, as mentioned by the author of De Sacramentis, II. 7 (written probably in the 5th or 6th century).