In 1301, Boniface VIII wrote Unam Sanctam in which he not only declared that there is no salvation outside the church, comparing the church to Noah's ark, but made the famous statement: "Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff." (Porro subesse Romano Pontifici omni humanae creaturae declaramus dicimus, definimus et pronunciamus omnino esse de necessitate salutis. (full text)) The context of the statement is the subjection of the temporal power of earthly kings to the supreme authority of the pope, as well as the necessity of the "Greeks" (i.e. the Eastern Orthodox) to treat the Bishop of Rome as the supreme earthly spiritual authority.
This statement illustrates one way in which Rome's gospel is not the apostolic gospel. The apostles never taught what Boniface VIII defines here. This is not an article of faith that was taught either explicitly or implicitly by the apostles, and consequently - even on Aquinas' definition of papal power - it was not within the pope's power to define this article of faith ("And since the Church is founded on faith and the sacraments, the ministers of the Church have no power to publish new articles of faith, or to do away with those which are already published, or to institute new sacraments, or to abolish those that are instituted, for this belongs to the power of excellence, which belongs to Christ alone, Who is the foundation of the Church. ") (source and additional discussion) It is not an article of faith taught by Scripture or one to be found among the teachings of the early church. Unam Sanctam quotes Scripture, to be sure, but it does so inappropriately.
For example, Boniface VIII states:
Therefore, of the one and only Church there is one body and one head, not two heads like a monster; that is, Christ and the Vicar of Christ, Peter and the successor of Peter, since the Lord speaking to Peter Himself said: 'Feed my sheep' [Jn 21:17], meaning, my sheep in general, not these, nor those in particular, whence we understand that He entrusted all to him [Peter]. Therefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John 'there is one sheepfold and one shepherd.'The use of "feed my sheep" without specifying which sheep does not imply that Peter was to feed every sheep. As Paul writes to the Galatians:
(For he that wrought effectually in Peter to the apostleship of the circumcision, the same was mighty in me toward the Gentiles:)
So we see from Scripture that Paul fed the Gentiles, while Peter ministered to the Jews.
Likewise, it is true that there is one shepherd, but that shepherd is not Peter, or his successors, but Christ himself. As it is written:
The LORD is my shepherd; I shall not want.
Give ear, O Shepherd of Israel, thou that leadest Joseph like a flock; thou that dwellest between the cherubims, shine forth.
Now the God of peace, that brought again from the dead our Lord Jesus, that great shepherd of the sheep, through the blood of the everlasting covenant,
Peter himself testifies:
1 Peter 2:25
For ye were as sheep going astray; but are now returned unto the Shepherd and Bishop of your souls.
1 Peter 5:4
And when the chief Shepherd shall appear, ye shall receive a crown of glory that fadeth not away.
And, of course, Jesus himself in the context explains who the one shepherd is:
I am the good shepherd: the good shepherd giveth his life for the sheep.
I am the good shepherd, and know my sheep, and am known of mine.
So, Boniface VIII is right that the church is not a two-headed monster, but the one head is Christ, not Boniface VIII. Peter was not the head of the church, and there is no unique successor of Peter - rather many have succeeded Peter in feeding Christ's sheep.
Indeed, Boniface VIII himself confessed earlier in the same short work:
We believe in her firmly and we confess with simplicity that outside of her there is neither salvation nor the remission of sins, as the Spouse in the Canticles [Sgs 6:8] proclaims: 'One is my dove, my perfect one. She is the only one, the chosen of her who bore her,' and she represents one sole mystical body whose Head is Christ and the head of Christ is God [1 Cor 11:3]. In her then is one Lord, one faith, one baptism [Eph 4:5].This ought to have informed Boniface VIII that Christ alone is the head, and he is not. But notice the strange apparent suggestion that there is one lord "in her". The "one Lord" that Paul is referring to in Ephesians 4:5 is Christ, not a lord in the church. As it is written:
1 Corinthians 8:6
But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him.
So, the apostolic faith is that the one Lord of the church is not "in her" but over her. But if someone will insist that Boniface VIII here meant to refer to Christ, not himself, all the worse for Boniface VIII's later statements!
Boniface also attempts an allegorical exegesis:
There had been at the time of the deluge only one ark of Noah, prefiguring the one Church, which ark, having been finished to a single cubit, had only one pilot and guide, i.e., Noah, and we read that, outside of this ark, all that subsisted on the earth was destroyed.But the captain of our salvation is not the bishop of Rome, but Christ himself:
For it became him, for whom are all things, and by whom are all things, in bringing many sons unto glory, to make the captain of their salvation perfect through sufferings.
And it is the Spirit of truth that guides us into all truth:
Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.
Moreover, Noah was not the pilot or guide of the ark, for we are told that it was an ark. It is nowhere described as having rudder or helm nor yet a keel. Thus, God alone was the guide and pilot of Noah's ark.
Thus, it is written:
And the waters prevailed, and were increased greatly upon the earth; and the ark went upon the face of the waters.
And the ark rested in the seventh month, on the seventeenth day of the month, upon the mountains of Ararat.
But Noah remained contained within the ark, so that he could not see to steer, if he had wished to:
And it came to pass at the end of forty days, that Noah opened the window of the ark which he had made:
And it came to pass in the six hundredth and first year, in the first month, the first day of the month, the waters were dried up from off the earth: and Noah removed the covering of the ark, and looked, and, behold, the face of the ground was dry.
We can easily add one final example of Boniface VIII's misuse of Scriptures in this document:
Therefore, if the terrestrial power err, it will be judged by the spiritual power; but if a minor spiritual power err, it will be judged by a superior spiritual power; but if the highest power of all err, it can be judged only by God, and not by man, according to the testimony of the Apostle: 'The spiritual man judgeth of all things and he himself is judged by no man' [1 Cor 2:15].Read the context in 1 Corinthians 2:7-16:
But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory. But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.So, here Paul is not describing a singular bishop of Rome, but rather he is referring to himself and the Corinthian believers. It is apparent, therefore, that Boniface VIII has wrenched this phrase about the spiritual man out of its proper context to make it refer uniquely to his office - an office that did not even exist in the time of Paul, as many of Rome's historians today acknowledge.
But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God. For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
Now we have received, not the spirit of the world, but the spirit which is of God; that we might know the things that are freely given to us of God. Which things also we speak, not in the words which man's wisdom teacheth, but which the Holy Ghost teacheth; comparing spiritual things with spiritual.
But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned. But he that is spiritual judgeth all things, yet he himself is judged of no man. For who hath known the mind of the Lord, that he may instruct him? but we have the mind of Christ.
I could go on and on, but surely the point has already been made. What Boniface VIII taught as being part of the gospel ("absolutely necessary for salvation") is a false gospel.