That's easy!
To quote Unam Sanctam, the bull which you love to quote out of context, the opening sentence is,
"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].
Catholics follow catholic popes, not heretics. There's been more than one pope in the history of the Catholic Church and one cannot obey two masters.
I've quoted nothing out of context. It is word for word from the papal encyclicals online. The last sentence is the summation of the bull.
Your quote illustrates one of the errors of Rome.....you believe in an organization over the Savior.
Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff.
http://www.papalencyclicals.net/bon08/b8unam.htm
You. Simply. Have. No. Choice.
So, who was the last legitimate pope?