After having gone through his course of theology, and taken the degree of Master of Arts, he was chosen—the same year, A.D. 1506—by the community of Glaris to be their pastor. There he remained for ten years, faithfully discharging his professional duties while diligently studying the Holy Scriptures. During this time he seems to have acquired in knowledge and experience the needed preparation for his future services to the Lord and to His church. "A most interesting manuscript," says one of his biographers, "still exists in the library of Zurich—a copy of all St. Paul's epistles in the original Greek, with numerous annotations from the principal fathers, which Zwingle wrote with his own hand, and then committed entire to memory." At the end of the MS. is written, "copied by Ulric Zwingle, 1514." He also studied the Latin classics, and collected from the writings of the fathers—especially from Origen, Ambrose, Jerome, Augustine, and Chrysostom—the doctrines and practices of the early church. "I study the doctors," he said, "not as authorities, but with the same end as when we ask a friend: How do you understand this passage?" The writings of Wycliffe and Huss he also knew, but like all students of his age, he devoured the writings of Erasmus as they successively appeared.
From this time, the ecclesiastical abuses which Rome had introduced became obvious to his mind; and, while expounding the scriptures from the pulpit, he faithfully and fearlessly exposed the innovations and corruptions of the Romish system. This was the dawn of the Reformation in Switzerland. Zwingle was maintaining the absolute authority of the truth of God and denouncing the falsehoods of Rome.
While thus engaged, he was obliged to leave his more sacred duties, and accompany the confederate army on an Italian expedition. Threatened by Francis I. who vowed to avenge in Italy the honor of the French name, the pope, in great consternation, entreated the cantons to come to his aid. It was then the custom in Switzerland for the Landamman, or chief magistrate of the canton, and the pastor of the parish to take the field with the troops on such campaigns. In the years 1513 and 1515, Zwingle was compelled to follow the banner of his parish to the plains of Italy. On the former of these occasions, the French were defeated by the confederates at Novara; and monks and priests proclaimed from their pulpits that the Swiss were the people of God, who avenged the bride of the Lord on her enemies. But, on the latter occasion he witnessed a signal defeat of his countrymen on the fatal field of Marignan. There, says history, the flower of the Helvetian youth perished. And Zwingle, who had been unable to prevent the great disaster, and overcome by his national feelings and patriotism, seized a sword and threw himself into the midst of danger. This was natural, and in those times it was considered noble, but it was not christian. He forgot for the moment that as a minister of Christ he should fight only with the sword of the Spirit which is the word of God. "For the weapons of our warfare are not carnal," says the apostle, "but mighty through God to the pulling down of strongholds; casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ." (2 Cor. 10:4, 54(For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;) 5Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ; (2 Corinthians 10:4‑5).)
Zwingle now felt more keenly than ever the necessity of reform in both church and state. He had seen the consequences of the practice long prevalent among his people, of letting out their soldiers to fight the battles of other nations, and to settle quarrels which did not belong to them. The sight of so many of his brave countrymen being slaughtered beyond the Alps in defense of a faithless and ambitious pope, filled him with indignation. He raised his voice against the practice; and, through his means, it was given up by several of the cantons. He also saw when in Italy, as Luther had seen, the pride and luxury of the prelates, the avarice and ignorance of the priests, and the licentiousness and dissipation of the monks. His future course was decided. He ascended the pulpit with a holier determination to preach the word of God more clearly, more fully, comparing scripture with scripture; and soon a fresh spirit of inquiry began to breathe on the mountains and in the valleys of Switzerland.
The question of priority between Zwingle and his Saxon ally, as to their aggressions on the papacy, has been raised by some historians Both seem to have received the truth about the same time, especially the knowledge of salvation by grace through faith alone; but as a Reformer, Luther evidently was first in the field. When Zwingle was preaching the gospel in a comparatively quiet way, Luther was publicly raising the standard of truth against the dominion of error, and causing his voice to be heard in all parts of Christendom.