Beatrice: "Eternal light of that great man
To whom Our Lord on earth bequeathed the keys, ...
This person test and try concerning faith,
By which thou once didst walk upon the seas." (Canto XXIV. 34-39)
Here Beatrice and Dante encounter the bright light of St. Peter and Beatrice asks Peter to test Dante's faith. The "keys" refers to Peter's spiritual power on earth and "walk upon the seas" refers to Peter's walking of the waters at the request of Christ and as we may recall sank when his faith diminished.
Peter: "What is faith?"
Dante: "The substance of things hoped for faith must be,
And argument of things invisible...." (Canto XXIV. 64-65)
Here Dante quotes from Paul's letter to the Hebrews 11:1which in most of today's translations reads, "Faith is the substance of what is hoped for and the evidence of things not seen". He goes on to explain that since we cannot see the mysteries of heaven with our eyes we must rely on faith which is the substance or foundation of hope and from which we can proceed toward a deeper understanding by reason; this is why, Dante states, it is called argument (evidence).
Dante follows up Peter's questioning by stating that he received his faith through the inspired books of the Old and New testament. Dante knows they are inspired because truths found there could not be the works of human nature and are backed up by miracles. But, but how does Dante know that these miracles really took place? He states,
"If without miracles the world did bide
By Christ, then all the miracles I know
Are by this one a hundredfold greater."
(Canto XXIV. 106-108)
At this point the heavenly souls of this heavenly sphere burst out in sung-
(Recorded at St. Benedict Monastery in São Paulo (Brazil)
(click Te Deum to hear)
Now St. Peter wants to know what Peter believes and what authority he grounds his faith.
"One God, eternal, sole, my creed doth know,
Mover of Heaven, being Himself unmoved;
Loving, desiring Him, around they go."
(Canto XXIV. 130-132)
Here it affirms that one of the proofs for his faith in God is given by science and metaphysics as exemplified by Aristotle who argued that "Since everything is moved by something and since motion is eternal, Aristotle concludes that there must be something that imparts motion without itself being moved; otherwise, there would be an infinite regress of movers, the moved and instruments of moving, which is unacceptable (Physics 8.5). (An axiom for Aristotle is that an infinite regress is impossible.) According to Aristotle, all movable things are only potentially in motion, and require something else to act upon them in order to be set in motion: "So it is clear that in all these cases the thing does not move itself, but it contains within itself the source of motionnot of moving something or of causing motion, but of suffering it." (Physics 8.4; 255b 29-31). Thus, if there were no unmoved mover, there could be no motion, because a moved mover requires a cause of its own motion and no infinite regress is possible." (Professor Barry D. Smith at www.abu.nb.ca/Courses/GrPhil/PhilRel/Aristotle.htm)
While for Dante there was no contradiction between reason and faith, reason is not faith but a prelude to faith. There is no doubt that man can come to know God by a simple intelligent awareness that the vast cosmic universe could not have come into being by itself. The philosopher instead come to know God by a rigorous process of reasoning. All of this knowledge does not constitute faith. Faith means a personal encounter with the living God which elicits an act of faith and to whom we turn in prayer.
Dante's faith is also based on on Biblical Revelation which not only affirms the One God, but as he states--
"As tenet of my faith; so One and Trine
That are and is their nature both denotes."
(Canto XXIV. 140-141)
Here again reason could not have discovered the inner essence of God reason and the reasonableness of revealed truth is not faith. 'What moves us to believe is not the fact that revealed truths appear as true and intelligible in the light of our natural reason: we believe 'because of the authority of God himself who reveals them, who can neither deceive nor be deceived.' So 'that the submission of our faith might nevertheless be in accordance with reason, God willed that external proofs of his Revelation should be joined to the internal helps of the Holy Spirit.' " (Catechism of the Catholic Church, No. 156)
"So, singing blessings then for my soul's sake,
As I was silent ...." (Canto XXIV. 151-152)
Canto XXV: Heaven of Fixed Stars, Dante's Hope