I’m not sure what you mean by “speak infallibly for God,” so I’ll just reproduce the language of the Catechism, and we can go from there:
*890 The mission of the Magisterium is linked to the definitive nature of the covenant established by God with his people in Christ. It is this Magisterium’s task to preserve God’s people from deviations and defections and to guarantee them the objective possibility of professing the true faith without error. Thus, the pastoral duty of the Magisterium is aimed at seeing to it that the People of God abides in the truth that liberates. To fulfill this service, Christ endowed the Church’s shepherds with the charism of infallibility in matters of faith and morals. The exercise of this charism takes several forms: *
891 “The Roman Pontiff, head of the college of bishops, enjoys this infallibility in virtue of his office, when, as supreme pastor and teacher of all the faithful - who confirms his brethren in the faith he proclaims by a definitive act a doctrine pertaining to faith or morals… The infallibility promised to the Church is also present in the body of bishops when, together with Peter’s successor, they exercise the supreme Magisterium,” above all in an Ecumenical Council.[418] When the Church through its supreme Magisterium proposes a doctrine “for belief as being divinely revealed,”[419] and as the teaching of Christ, the definitions “must be adhered to with the obedience of faith.”[420] This infallibility extends as far as the deposit of divine Revelation itself.[421]
The question is whether the assumption is reasonable. If you want to send a message reliably, what’s the best way to do it? Would you leave only a textbook, or would you also demonstrate to people what you were trying to teach? Clearly, the best way to preserve meaning would be to do the latter. I can’t understand why it is questionable to think that God would understand something that every educational system in human history has grasped: that passing knowledge through both education and writing is the most effective way to preserve meaning.
Yes, but you’re talking with Catholics, and the only operative question is whether that claim has any strength when you conflict with Catholic belief. Asserting the principle is meaningless unless you ground it in concrete cases discernible from the historical record to assess whether the claim of infallibility is robust. In every instance where sola scriptura has been put to the test against a doctrine that one would expect from the historical record to be apostolic in origin, it has failed. Now tell me, why would one believe anything that is wrong in every single historical instance in which it has been put to the test? It’s just a meaningless platitude to say that Scripture is “the only infallible authority” when any meaningful test that would even make that claim believable turns out the wrong way.