Explaining the Keys in Matthew 16:19
Everyone has a tendency toward being close-minded when it comes to learning about dying to oneself. So, obviously the Papacy will be problematic for the Protestant. The effects of Original Sin, that is, weakened will and darkened intellect, make it even more difficult. When it comes to learning about religious truth that a person doesn’t already believe most people are about 90 % closed-minded and the other 10 % is closing fast.
One difficulty in using Isaiah 22 to explain the meaning of the Keys is the unfamiliar terms of "master of the palace" or harder still other translations like "over the house." These unfamiliar terms leave a very foggy notion of what is meant. "Master of the palace" sounds more like the king himself than some other officeholder. But if it is the king then his position is replaced by Christ’s.
And unfortunately only clear thinking will lead to that certainty of conviction that will enable a person to die to himself. [That is why heterodox teaching is so dangerous. The faithful are endangered not only when the faith is attacked outright but also when doubt is cast upon the truths of the Good News.] When hearing these terms, i.e. master of the palace, the typical Protestant’s mind is quickly closing shut and it moves onto another subject before he really examines the power of these Scriptural verses. This fact will work to the Catholic’s disadvantage when he starts with Matthew 16:19 and then goes to Isaiah 22 to explain the meaning of the keys.
Also, Catholics need to help Protestants overcome their "either / or," that is, follow Christ or Peter misunderstanding, and help them accept Christ and how Christ works through St. Peter.
It is much better, in my opinion, to start with the Old Testament and explain who is the "master of the palace" and thereby the meaning of the keys, and only after that go into the subject of the Papacy. By explaining the relationship between King Hezekiah and his chief minister Eliakim in Old Testament first, the ground work is laid for the proper understanding of the relationship between Christ and St. Peter.