According to Pope Francis and/or Catholicism, what sins are more serious than sins of the flesh?

Atheism (sin of unbelief in God) is the greatest sin of all because there is nothing more hateful of God than to believe He does not exist.

As St. Thomas Aquinas writes in Summa Theologica II-II q. 10 a. 3 ("Whether unbelief is the greatest of sin?") co.:

Every sin consists formally in aversion from God […] Hence the more a sin severs man from God, the graver it is. Now man is more than ever separated from God by unbelief, because he has not even true knowledge of God: and by false knowledge of God, man does not approach Him, but is severed from Him.

^{from this answer to "Do Catholics view atheism as a sin?"}

The worst sin you can possibly commit is to refuse to go to Mass when you are required. Why? Because we, as creatures, owe God worship. To fail to attend Mass is to refuse to worship God. The highest form of worship is sacrifice and the sacrifice of the Mass is a participation in the body and blood of the Lord, as the Greek word anamnesis in Luke's last supper account and St. Paul's use in 1 Cor 10-11 indicate. What the word means is lost in translation because most translations usually record it as something like, "do this in remembrance of me." The word remembrance is the translation of the Greek which actually mean to remember and in the recall, to make it present again, 'it' being the sacrifice of Jesus on Calvary.

So, if you summed up his other statements and works where does it seem that the Holy Father ranks sins of the flesh and how does that compare to the traditional teaching of the Catholic Church?

Schism is the most serious sins, it is more than a mortal sin, because, one who embraced schism will eventually incur "automatic excommunication" by virtue of Canon751 and Canon1364., and can no longer avail of the Sacraments, esp. the Holy Eucharist and the Sacraments of Confession.

Baptized Catholic who embraced schism is not in full communion with the Church and most do not agree on approved Magisterial Teachings of Pope Francis. And this schismatic behavior, according to Church teaching, a single teaching that a Catholic reject and oppose, they must not receive the Holy Eucharist.

Schism is both a mortal sin and a canonical crime, while sin of the flesh can, if it just a waekness of the flesh like St.Paul experienced and teaches, is not really a big obstacle or a sin that will deprive the offender to avail of the Holy Eucharist and the Sacrament of Confession, because the sinner is still inside the Church, unlike the schismatic.

Can. 751 Heresy is the obstinate denial or doubt, after baptism, of a truth which must be believed by divine and catholic faith. Apostasy is the total repudiation of the christian faith. Schism is the withdrawal of submission to the Supreme Pontiff or from communion with the members of the Church subject to him.

Can. 1364 §1. Without prejudice to the prescript of can. 194, §1, n. 2, an apostate from the faith, a heretic, or a schismatic incurs a latae sententiae excommunication; in addition, a cleric can be punished with the penalties mentioned in can. 1336, §1, nn. 1, 2, and 3.

And since, numerous Traditionalist affected by Traditionis Custodes are expressing publicly in media, esp, in the social media, evil things and accusatory words on the Church Magisterium, then another Canon Law can be violated.

Can. 1369 A person who in a public show or speech, in published writing, or in other uses of the instruments of social communication utters blasphemy, gravely injures good morals, expresses insults, or excites hatred or contempt against religion or the Church is to be punished with a just penalty.

In closing, schism is the most grievous sin that sin of the flesh, because this sin belongs to Lucifer, and this sin, is the cause of Lucifer cast out in the Heavenly realms. Also, schism is a form of rigidity.

"Rigidity is a sin against the patience of God."-Pope Francis

According to Pope Francis and/or Catholicism, what sins are more serious than sins of the flesh?

Obviously some sins are more serious than others. The Church has always taught that some sins are venial; some sins are mortal and there is an unforgivable sin.

A venial sin does not total destroy our friendship with God and is easily forgiven by asking God’s forgiveness with true sincerity.

Venial Sin

An offense against God which does not deprive the sinner of sanctifying grace. It is called venial (from venia, pardon) because the soul still has the vital principle that allows a cure from within, similar to the healing of a sick or diseased whose source of animation (the soul) is still present to restore the ailing bodily function to health.

Deliberate venial sin is a desease that slackens the spiritual powers, lowers one's resistance to evil, and causes one to deviate from the path that leads to heavenly glory. Variously called "daily sins" or "light sins" or "lesser sins," they are committed under a variety of conditions: when a person transgresses with full or partial knowledge and consent to a divine law that does not ablige seriously; when one violates a law that obliges gravely but either one's knowledge or consent is not complete; or when one disobeys what is an objectively grave precept but due to invincible ignorance a person things the obligation is not serious.

The essence of venial sin consists in a certain disorder but does not imply complete aversion from humanity's final destiny. It is an illness of the soul rather than its supernatural death. When people commit a venial sin, they do not decisively set themselves on turning away from God, but from overfondness for some created good fall short of God. They are like person who loiter without leaving the way.

Mortal sin will destroy our friendship with God and normally can only be forgiven through sacramental confession or by a perfect act of contrition when a priest is unavailable.

Mortal Sin

An actual sin that destroys sanctifying grace and causes the supernatural death of the soul. Mortal sin is a turning away from God because of a seriously inordinate adherence to creatures that causes grave injury to a person's rational nature and to the social order, and deprives the sinner of a right to heaven.

The terms mortal, deadly, grave, and serious applied to sin are synonyms, each with a slightly different implication. Mortal and deadly focus on the effects in the sinner, namely deprivation of the state of friendship with God; grave and serious refer to the importance of the matter in which a person offends God. But the Church never distinguishes among these terms as though they represented different kinds of sins. There is only one recognized correlative to mortal sin, and that is venial sin, which offends against God but does not cause the loss of one's state of grace. (Etym. Latin mors, death.)

Then there is the question of the unforgivable sin! Refusing the grace of repentance is a greater sin than those of the flesh. One’s pride keeps us from humbly asking God’s forgiveness at the moment of dying.

Therefore I say to you: Every sin and blasphemy shall be forgiven men, but the blasphemy of the Spirit shall not be forgiven.

And whosoever shall speak a word against the Son of man, it shall be forgiven him: but he that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world, nor in the world to come. Matthew 12:31-32

But what is this unforgivable sin?

This probably is a reference to the following Gospel passage: “Therefore, I say to you, every sin and blasphemy will be forgiven people, but blasphemy against the Spirit will not be forgiven. And whoever speaks a word against the Son of Man will be forgiven; but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come” (Matthew 12:31-32). Here is how the Catechism explains this passage: “There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit. Such hardness of heart can lead to final impenitence and eternal loss” (CCC 1864).

The basic idea is simple. God’s mercy is infinite. He is always willing and able to forgive us. But he won’t force his forgiveness upon us. If we refuse to accept the grace that God offers us (and this grace is always offered through the Holy Spirit) then we cannot benefit from that grace. We are free to refuse the gifts that God offers us. That is the “unforgivable sin.” But it is not unforgivable because God’s mercy is limited. Rather, it’s unforgivable because of our refusal to accept God’s forgiveness. The faucet of God’s mercy always works, so to speak, but to get the cleansing water we need to turn it on.

It is worth recalling that we may reject God’s action or turn away from him at one point in our lives, and then repent and turn back to him later. As soon as we turn back, humbly repentant and seeking forgiveness and reconciliation, God’s mercy will touch and renew us. The only way that we will be forever separated from God’s love is if we stubbornly and consistently for post on the unforgivable sinrefuse the invitations of his grace (the action of the Holy Spirit in our lives) up to the moment of our death. Here again is how the Catechism puts it: “God predestines no one to go to hell; for this, a willful turning away from God (a mortal sin) is necessary, and persistence in it until the end” (CCC 1037).

Practically, this means that we all must humbly and joyfully continue to seek a deeper knowledge of God and a constant obedience to his commandments and inspirations. For this, prayer, study, the sacraments (especially the Eucharist and Confession), and good Christian fellowship are all critical. If you want to reflect a little bit more on the reality of sin in general, you may find my video conference on this topic useful. It is called “The Anatomy of Sin.”

• What Is The Unforgivable Sin?

In the end, it is obvious that Pope Francis is saying that there are sins that are venial and we should be on our guard that these do not develop into something more serious such as mortal sin.

Please bare in mind what St. Peter, the first Pope said in his epistle.

Be sober and watch: because your adversary the devil, as a roaring lion, goeth about seeking whom he may devour. - 1 PETER 5:8

Traditionally, Pride and Envy are thought to be the gravest of sins, because they are spiritual sins. Thomas Aquinas has a number of articles in a question in the Summa comparing sins. Article 5 is of importance here.

I answer that, Spiritual sins are of greater guilt than carnal sins: yet this does not mean that each spiritual sin is of greater guilt than each carnal sin; but that, considering the sole difference between spiritual and carnal, spiritual sins are more grievous than carnal sins, other things being equal. Three reasons may be assigned for this. The first is on the part of the subject: because spiritual sins belong to the spirit, to which it is proper to turn to God, and to turn away from Him; whereas carnal sins are consummated in the carnal pleasure of the appetite, to which it chiefly belongs to turn to goods of the body; so that carnal sin, as such, denotes more a "turning to" something, and for that reason, implies a closer cleaving; whereas spiritual sin denotes more a "turning from" something, whence the notion of guilt arises; and for this reason it involves greater guilt. A second reason may be taken on the part of the person against whom sin is committed: because carnal sin, as such, is against the sinner's own body, which he ought to love less, in the order of charity, than God and his neighbor, against whom he commits spiritual sins, and consequently spiritual sins, as such, are of greater guilt. A third reason may be taken from the motive, since the stronger the impulse to sin, the less grievous the sin, as we shall state further on (Article 6). Now carnal sins have a stronger impulse, viz. our innate concupiscence of the flesh. Therefore spiritual sins, as such, are of greater guilt.