R.C. Sproul writes about Luther's insistence on asserting the truth of Scripture and with great conviction and confidence.
"'Nothing is more familiar or characteristic among Christians than assertion. Take away assertions, and you take away Christianity.' Then, in his passion, Luther said: 'Away, now, with Skeptics and Academics from the company of us Christians; let us have men who will assert.'"
"Luther would have none of the spirit of those who are always learning and never coming to a knowledge of the truth (2 Tim. 3:7). The foundational truths of Christianity were built on the blood of the martyrs, because the apostles didn't go into the marketplace saying: 'Well, maybe Jesus rose from the dead or maybe He didn't. You need to examine this, and suspend judgment.' No, they were bold in their assertions because they know what they believed (2 Tim. 1:8-12). They understood the things of God, were convinced of the truth of the claims of Jesus, and, having that assurance and certainty of the truthfulness and trustworthiness of Scripture, they went boldly into a dying world. Luther did the same."
Feed My Sheep. Sproul. 77