21 And would you be sure that by the power of the Holy Ghost the Martyrs bear their witness? The Saviour says to His disciples, And when they bring you unto the synagogues, and the magistrates, and authorities, be not anxious how you shall answer, or what you shall say; for the Holy Ghost shall teach you in that very hour, what you ought to say. For it is impossible to testify as a martyr for Christ's sake, except a man testify by the Holy Ghost; for if no man can say that Jesus Christ is the Lord, but by the Holy Ghost, how shall any man give his own life for Jesus' sake, but by the Holy Ghost?
Source: Catechetical Lectures (New Advent)