The same day he nailed his 95 theses to the door of Castle Church in Wittenberg, Martin Luther wrote a letter to Cardinal Albrecht, the Archbishop of Mainz (October 31, 1517). In it he writes:

Works of piety and love are infinitely better than indulgences; and yet [the indulgence preachers] do not preach them with an equally big display and effort. What is even worse, [the preachers] are silent about them because they have to preach the sale of the indulgences. The first and only duty of the bishops, however, is to see that the people learn the gospel and the love of Christ. For on no occasion has Christ ordered that indulgences should be preached, but he forcefully commanded the gospel to be preached. What a horror, what a danger for a bishop to permit the loud noise of indulgences among his people, while the gospel is silenced, and to be more concerned with the sale of indulgences than with the gospel! Will not Christ say to [such bishops], “You strain out a gnat but swallow a camel”?

[Source: Luther’s Works, vol. 48, page 47.]