Therefore he who would find Christ must first find the Church. How should we know where Christ and his faith were, if we did not know where his believers are? And he who would know anything of Christ must not trust himself nor build a bridge to heaven by his own reason; but he must go to the Church, attend and ask her. Now the Church is not wood and stone, but the company of believing people; one must hold to them, and see how they believe, live and teach; they surely have Christ in their midst. For outside of the Christian church there is no truth, no Christ, no salvation.-Martin luther: Sermon for the Early Christmas Service; Luke 2:15-20 (1521-1522). Luther's Works, American Ed., Hans J. Hillerbrand, Helmut T. Lehmann ed., Philadelphia, Concordia Publishing House/Fortress Press, 1974, ISBN 0-8006-0352-4 (Sermons II), vol. 52:39-40
Why would you not know? The scripture you listed confirmed it. “For outside of the Christian church there is no truth, no Christ, no salvation.” Protestants are in a Christian Church so they are not outside the Church.
Any Christian Church that believes that their church is the only way to salvation is flawed and offensive. I would think a Catholic would be offended as well if a Protestant told them they were not sure Catholics could attain salvation (not my belief BTW).