The one who comes from above is above all; the one who is of the earth belongs to the earth and speaks about earthly things. The one who comes from heaven is above all.  He testifies to what he has seen and heard, yet no one accepts his testimony.  Whoever has accepted his testimony has certified this, that God is true.  He whom God has sent speaks the words of God, for he gives the Spirit without measure.  The Father loves the Son and has placed all things in his hands.  Whoever believes in the Son has eternal life; whoever disobeys the Son will not see life, but must endure God's wrath.

Belief is the submission of the intellect to God, obedience is the submission of the will to God. Both are necessary for salvation because both constitute the entire human person and it is the entire human person which must submit to Christ.

The disharmony brought about by original sin introduced a rift in man such that the will does not always conform to the intellect and the intellect does not always conform to the truth. The Church Fathers interpret the words "Christ is above all" as meaning in part that Christ's human soul and nature is that of man's before the fall. (*) Therefore can we not conclude that in order to be saved, in order to enter into the mystery of salvation from original sin, the whole person must submit themselves to Christ in order that their intellect and will be knitted back together by Christ's perfect nature, thus allowing us to enter Paradise by being in Him? Can this not also be seen as a true metaphysical reality and thus an acknowledgment, a certification, by us that "God is true?"

The man who holds that faith alone saves has given an intellectual assent to an enterprise in which his heart is not necessarily in it. It does not come at the cost of the suffering of obedience; the destruction of self-will and self-reign. He neglects the Scripture which says "even the demons believe - and shudder." (James 2.19). The one who proceeds on faith alone cannot enter Paradise because his fallen will would not be healed in Christ.

The man who holds that works can save makes himself God and empties the cross of its meaning. He reduces Christ to a teacher whose purpose it was to tell us which works to perform in order to be judged worthy of heaven. He neglects the Scripture which says " 'Lord open to us. We ate and drank with you, and you taught in our streets.' But the Lord will say, 'I do not know where you come from.' " (Luke 13.25) The acting man who has not performed the assent of faith cannot enter Paradise because his fallen intellect would not be healed in Christ.

Heaven has only one door - Jesus Christ. It is only through believing in him and obeying him that a man can enter heaven through him. This is a metaphysical necessity that recognizes the rift between man's intellect and will and the redemption of that rift in the Incarnate Word who assumed a pre-Fall human soul and nature (*), a perfectly united human intellect and will, such was his great love, thus opening the way into Paradise for the one who believes and obeys him, the one who thereby becomes one healed man in Him, by means of Him, and for Him.

* Important note

The reference here is from Augustine referencing Alcuin, as quoted by St. Thomas Aquinas, expounding John 3.31: " 'Christ came from above, i.e., from the height of human nature, which it had before the fall of the first man.'  For He received human nature without sin, in the purity which it had in the state of innocence.  In the same way He might have assumed human nature without defects.  Thus it is clear that Christ did not contract these defects as if taking them upon Himself as due to sin, but by His own will." (Summa Theologica, III, Q14, article 3).  Christ was subject to some human infirmities and defects (death, hunger, thirst and the like) because he assumed the penalties due to our sin.  Also, because he otherwise would not have seemed to be true man.  However there are some defects in human nature which Christ did not have because they are incompatible with the perfection of knowledge and grace, such as ignorance, a proneness towards evil, and a difficulty in well-doing.  The defects Christ did have he assumed by his own will, not because of possessing a sinful human nature.  This is my reading of St. Thomas Aquinas, Summa Theologica, Third Part, Question 14.  But as in all things I submit to the teaching authority of the Holy Catholic Church if I misread this.