In this post my aim is to present a brief biblical defense of the doctrine of ecclesial infallibility. I do not intend to be particularly technical: what I mean for the purposes of this post by “the doctrine of ecclesial infallibility” is that Christ’s Church must exercise infallibility on some terms or other, under certain conditions or other. The necessity of this is inescapable based upon at least three passages of Scripture. There is no particular need for us to consider them one at a time, so let’s see what the Bible says.
Matthew 16:19: “I will give you the keys of the kingdom of Heaven: whatever you bind on earth will be bound in heaven; whatever you loose on earth will be loosed in heaven.”
Matthew 18:18: “In truth I tell you, whatever you bind on earth will be bound in heaven; whatever you loose on earth will be loosed in heaven.”
John 20:23: “If you forgive anyone’s sins, they are forgiven; if you retain anyone’s sins, they are retained.”
In the first of these passages, our Lord is speaking to St. Peter; in the other two he is speaking to all the apostles.
The initial thing we should notice is that all three of these passages contains a promise: Jesus promises something to them, and He does so unconditionally: “whatever you bind;” “anyone’s sins.” We must of course keep this fact in mind, but what is far more important (obviously) is what He promises. In effect, Jesus guarantees that He will honor what St. Peter binds or looses, and that He will honor what the apostles together bind or loose, and that He will forgive (or not) the sins of anyone that the apostles forgive (or not). Consider the enormity of this: God the Son is binding Himself to honor decisions made by His Church! This is no small thing; it is simply huge, and entails what can only be described as an authority granted to the Church that is eternal in its scope.
Now there is at least one question that must be asked: what if St. Peter and/or the apostles make a mistake, or what if they sin in the exercise of the authority the Lord has given to them? The answer to this question tells us why infallibility in some form is an essential attribute of Christ’s Church. For if they make a mistake in a decision subject to His promise, God has put Himself in the impossible position of confirming an error! Worse yet, if they sin in such a decision then He has made Himself complicit in their sin! What shall we say to this?
There is only one thing to be said: these are literally impossible circumstances. God does not err; God does not sin nor endorse evil. These facts being so, we are forced inexorably toward the only other tenable explanation: that is that God protects the leaders of His Church from error under at least some circumstances. I put it to you that no other reading of these passages does justice to what they say. And because this is the case, we must conclude that there are at least some conditions or circumstances of some sort in which the Church is protected by God from error, which is the same as saying that there are conditions under which the Church speaks with infallible authority.
Okay, so what conclusions may we draw from this obviously biblical fact? Well, among other things we must surely conclude that any ecclesial group which denies infallibility to Christ’s Church cannot itself be speaking with Christ’s authority. In short, it’s no more than someone’s opinion (one that happens to be erroneous) and we must ignore it (or refute it, as the case may be) as mistaken. Scripture gives us no other choice.
Obviously more could be said, but for the moment I will allow you, dear reader, to consider what other consequences might follow from what the Bible says about this subject.
Aquinas, Etc. 
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