Zwingli and Luther openly pressed a Reformation long secretly begun — the indulgence market drove them
Yet the Reformation itself had already been growing in secret for many ages — Flacius's Catalog of Witnesses of the Truth and James Ussher of Armagh in his golden booklet on church succession have said much about that hidden work.
Under God's lead it broke into the open in upper and lower Germany through Ulrich Zwingli and Martin Luther.
Zwingli led first by public teaching and by thundering against papal doctrine; Luther led first by striking the same targets with public writings and deeds of many kinds.
What chiefly dragged both men into the fight was the shameless papal indulgence-market, midwifed in one place by a certain Samsonius and in the other by infamous Tetzel (Gerhard Theodor Mejer, History of Religion, ch. V, §29, pp. 253–254).
Latin
Reformationem verò ipsam, multis jam retrò Seculis occultiùs inchoatam fotamque (quô de multi multa, ex professo Auctor Catalogi Testium Veritatis & Magnus Usserius Armachanus in aureo de Successione Ecclesiarum Libello) apertiùs, Deo duce, aggressi sunt in superiori alter, alter in inferiori Germania, Ulricus Zwinglius, & Martinus Lutherus; iste publicè docendô & contra papales doctrinas detonandô prior; hic easdem & scriptis & factis publicis variis prior impetendô impugnandóque; uterque, per impudentissimam cumprimis Indulgentiarum Papalium nundinationem, cui isthìc Samsonius quispiam, hic famosus Tezelius obstetricabantur, ad id pertractus, vid. Gerhard. Theod. Mejerum, Hist. Relig. Cap. V. §. 29. p. m. 253. 254.