Pope Boniface VIII, in Unam Sanctam, stated: “Now, therefore, we declare, say, define, and pronounce that for every human creature it is altogether necessary for salvation to be subject to the authority of the Roman pontiff.”Clearly Turretinfan has unfortunately divorced a very small portion of the papal text from the rest of the bull, making it into a pretext. I should note that this particular article of Turretinfan's has also been published on Dr. White's Alpha and Omega Blog here. I do wonder whether Mr. Turretinfan took the time to read the entire bull before publishing his article. If he would have, he would have found no need to write what he did. I'd like to ask the reader to take a few moments and read the entirety of Pope Boniface's Unam Sanctum before we continue in our examination of Turretinfan's article.
Scripture, in contrast, gives universal jurisdiction to Christ alone: “the blessed and only Potentate, the King of kings, and Lord of lords” (1 Timothy 6:15) and the Psalmist declares: “Thou madest him to have dominion over the works of thy hands; thou hast put all things under his feet” (Psalm 8:6) which the Apostle tells us plainly refers to Christ: “And hath put all things under his feet, and gave him to be the head over all things to the church” (Ephesians 1:22).
Urged by faith, we are obliged to believe and to maintain that the Church is one, holy, catholic, and also apostolic. 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]. 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.
We venerate this Church as one, the Lord having said by the mouth of the prophet: 'Deliver, O God, my soul from the sword and my only one from the hand of the dog.' [Ps 21:20] He has prayed for his soul, that is for himself, heart and body; and this body, that is to say, the Church, He has called one because of the unity of the Spouse, of the faith, of the sacraments, and of the charity of the Church. This is the tunic of the Lord, the seamless tunic, which was not rent but which was cast by lot [Jn 19:23-24]. 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.' We are informed by the texts of the gospels that in this Church and in its power are two swords; namely, the spiritual and the temporal. For when the Apostles say: 'Behold, here are two swords' [Lk 22:38] that is to say, in the Church, since the Apostles were speaking, the Lord did not reply that there were too many, but sufficient. Certainly the one who denies that the temporal sword is in the power of Peter has not listened well to the word of the Lord commanding: 'Put up thy sword into thy scabbard' [Mt 26:52]. Both, therefore, are in the power of the Church, that is to say, the spiritual and the material sword, but the former is to be administered _for_ the Church but the latter by the Church; the former in the hands of the priest; the latter by the hands of kings and soldiers, but at the will and sufferance of the priest.
However, one sword ought to be subordinated to the other and temporal authority, subjected to spiritual power. For since the Apostle said: 'There is no power except from God and the things that are, are ordained of God' [Rom 13:1-2], but they would not be ordained if one sword were not subordinated to the other and if the inferior one, as it were, were not led upwards by the other.
For, according to the Blessed Dionysius, it is a law of the divinity that the lowest things reach the highest place by intermediaries. Then, according to the order of the universe, all things are not led back to order equally and immediately, but the lowest by the intermediary, and the inferior by the superior. Hence we must recognize the more clearly that spiritual power surpasses in dignity and in nobility any temporal power whatever, as spiritual things surpass the temporal. This we see very clearly also by the payment, benediction, and consecration of the tithes, but the acceptance of power itself and by the government even of things. For with truth as our witness, it belongs to spiritual power to establish the terrestrial power and to pass judgement if it has not been good. Thus is accomplished the prophecy of Jeremias concerning the Church and the ecclesiastical power: 'Behold to-day I have placed you over nations, and over kingdoms' and the rest. 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]. This authority, however, (though it has been given to man and is exercised by man), is not human but rather divine, granted to Peter by a divine word and reaffirmed to him (Peter) and his successors by the One Whom Peter confessed, the Lord saying to Peter himself, 'Whatsoever you shall bind on earth, shall be bound also in Heaven' etc., [Mt 16:19]. Therefore whoever resists this power thus ordained by God, resists the ordinance of God [Rom 13:2], unless he invent like Manicheus two beginnings, which is false and judged by us heretical, since according to the testimony of Moses, it is not in the beginnings but in the beginning that God created heaven and earth [Gen 1:1]. Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff.
I have emphasized the important points which make up the totality of the bull's meaning. Boniface himself admits that the Head of the Church is indeed Christ, thus re-affirming 1 Tim. 6:15, Psalm 8:6, and Eph. 1:22. These passages are essential to the doctrine of the Papacy. But how is one actually subject to the Roman Pontiff? Answer: the exact same way one is subject to the Church. Being subject to the Christian Church, that same person is by that very fact made subject to the Roman Pontiff. Boniface doesn't say one must know he is subject to the Pope, but rather that he must be subject. How does this happen? Once again, the same way one becomes subject to the Church. By what means is this? By Holy Baptism, through which even infants become subject to the Church, and thus, to the Pope.
“The Church, guardian of the integrity of the Faith – which, in virtue of its authority, deputed from God its Founder, has to call all nations to the knowledge of Christian lore, and which is consequently bound to watch keenly over the teaching and upbringing of the children placed under its authority by baptism” - Pope Leo XIII, Nobilissima
“…Jesus Christ laid upon His Apostles the injunction to ‘preach the Gospel to every creature,’ He imposed, it is evident, upon all men the duty of learning thoroughly and believing what they were taught. This duty is intimately bound up with the gaining of eternal salvation: ‘He that believeth and is baptized shall be saved; but he that believeth not shall be condemned.’ [Mk. 16:16] But the man who has embraced the Christian faith, as in duty bound, is by that very fact a subject of the Church as one of the children born out of her, and becomes a member of that greatest and holiest body, which it is the special charge of the Roman Pontiff to rule with supreme power, under its visible head, Jesus Christ.” - Pope Leo XIII, Sapientiae christianae
“Moreover, if anyone without repentance dies in mortal sin, without a doubt he is tortured forever by the flames of eternal hell. – But the souls of children after the cleansing of baptism, and of adults also who depart in charity and who are bound neither by sin nor unto any satisfaction for sin, at once pass quickly to their eternal fatherland.” - Pope Innocent IV, Council of Lyons I
“If anyone shall say that infants, because they have not actual faith, after having received baptism are not to be numbered among the faithful… let him be anathema.” - Trent, Canon 13 on Justification
I think these few quotations will suffice for understanding in context the exact meaning of being subject to the Roman Pontiff. If Boniface's infallible statement was to be understood in the distorted way Protestants misunderstand it, one wonders why the Council of Trent didn't jump on this opportunity to reiterate it as dogma concerning salvation. They had no need to do so because no amount of documents one might sign saying they are subject to the Pope, even if they were signed in blood, would actually make that same person subject to the Pope. Only Holy Baptism or the Desire thereof makes such a person a son or daughter of the Church, and thus, subject to the Church's visible Pastor, the Roman Pontiff.
Hopefully this has been somewhat useful to you. I hope Mr. Turretinfan will correct his error in misunderstanding this Roman Catholic doctrine.