In short, let everyone “test,” that is, be diligently concerned that his ministry be faithful; for this above all is required in ministers of the Word (1 Cor. 4:2). It is as though [Paul] were saying: “Let everyone strive to achieve this, that he preach the Word purely and faithfully; and let him consider nothing except the glory of God and the salvation of souls. Then his work will be good in a faithful and solid way, and in his conscience he will have his boast, the sort of boast that can say with confidence: ‘This doctrine and my ministry are pleasing to God.’ That is truly a great and excellent boast.”
Luther’s Works, Volume 27, Page 119. Emphasis mine.