Boniface did not misspeak, but what he said is often misunderstood. Catholic Answers Staff Apologist Jimmy Akin laid out the basics this way:
The Necessity of Being Catholic (Condensed)
by James Akin
chnetwork.org/journals/nesschurch/ness_7.htm
- To be saved it is necessary to be a Christian.
- To be a Christian it is necessary to be a member of Christ’s Church.
- To be a member of Christ’s Church it is necessary to be a member of the Catholic Church.
- To be a member of the Catholic Church it is necessary to be subject to the Roman Pontiff.
- Therefore, it is necessary for salvation to be subject to the Roman Pontiff.
In this argument, the necessities are all normative necessities and the kind of membership being discussed is formal membership. The argument has a logically valid form (in fact, it expresses a variation on what is known as the “hypothetical syllogism” argument form), meaning that the truth of its conclusion depends only on the truth of the premises it contains.
When a Protestant objects to the above argument, it will be to the third proposition – that the Catholic Church is the Church Christ founded. Both sides agree on the other three points. While it is beyond the scope of this article to give a full proof of the third proposition (this is one of the major tasks of Catholic apologetics), we can offer a limited proof.
Both Protestants and Catholics agree that Christ founded some Church and that this Church will remain forever (Matt. 16:18). The question is whether this Church is a visible communion that can be identified or whether is it a purely spiritual communion made up of all the saved. If it is a visible communion, the Catholic Church is the only plausible candidate, since only this Church extends back far enough. We can thus give a limited argument for the third proposition by showing the Church Christ founded is a visible communion.
This is proven in Matthew 16:17-19, the passage in which Christ promised the gates of hell would never prevail against his Church (meaning that it would always exist). Several factors in the text show he was talking about a visible communion.
First, Jesus made Peter head of this Church (Matt. 16:18), yet Jesus was certainly not making Peter the head of an invisible Church. It is Christ’s own prerogative to be head of the invisible communion of Christians stretching from heaven to earth (Eph. 5:23). Therefore, he must have made Peter the head of a visible, earthly church. (We will not argue here that Jesus made Peter the head; even if one disagrees, the remaining arguments prove our case.)
Second, Jesus gave Peter the keys to the kingdom of heaven (Matt. 16:19), which are for use in Church government (compare Isa. 22:22 – the only Old Testament parallel to this verse). But one cannot govern an invisible communion of believers, only a visible one.
Third, Jesus gave Peter the power of binding and loosing (Matt. 16:19), which Matthew 18:17-18 indicates is used in Church discipline. But one cannot exercise Church discipline over an invisible body. Indeed, Matt. 18:17-18 refers it to public excommunication, in which an individual is treated by the church as “a gentile or a tax collector” (that is, as an unbeliever).
Fourth, Jesus explicitly stated that Peter would exercise the power of binding and loosing on earth. This shows his authority is an earthly one, over an earthly Church.
Fifth, Jesus promised the gates of hell would not prevail against his Church (Matt. 16:18), meaning that it would never perish. But it would be ridiculous to promise that an invisible Church would not pass out of existence since some of the Church’s members are in heaven and Christ’s heavenly Church cannot pass away by its very nature. Only a visible, earthly communion needs a promise that it will never perish.
There are thus abundant reasons to conclude that the Church Jesus was discussing in Matthew 16:17-19 was a visible communion of believers, and, since only the Catholic Church goes back that far, only it can be the one Christ founded.
+++
Of course, if Jimmy were presenting this material to an Orthodox audience, I’m sure he would add supporting materials that would address Orthodox objections.