For the reformers, the first mark of the true church was the faithful preaching of the Word of God. Lutheran pastor and theologian Johannes Brenz (1499-1570) argues in this excerpt from his commentary on Galatians that as a minister of the Lord, a true church leader is distinguished by the ability to speak the truth of the gospel without bending the message to satisfy human desires or ends.
Compromising the Gospel Is a Sin
People interpret this verse in different ways. The simplest explanation of it is that in these words Paul is excusing the sort of arrogance that his claims and persistence seem to imply. It is as if he had said, “I have preached my gospel with such authority and persistence that I have dared to condemn not only people but even angels who say anything different to punishment and hell.” In this verse we find an explanation of the duties of a true apostle and church leader, indeed of every individual Christian. First of all, an apostle or church leader must be so certain of his doctrine that he will not bend even to an angel . . . We realize that this certainty does not come from idle speculation apart from the Word of God but from the careful study of sacred Scripture by the Holy Spirit. The mark of a true church leader is that he preaches the truth of the gospel without any regard to human favor, power or wisdom. As Paul says, “If I try to please men I am not a servant of Christ.”
Explanation of Galatians, commenting on Galatians 1:10.
Galatians, Ephesians, ed. Gerald L. Bray. Reformation Commentary on Scripture, NT vol. 10, p. 32.