two points to this debate... contradictions in the Catholic church and lack of evidence and contraindications in the early church for infallibility.
CONTRADICTIONS
an infallible church can't contradict itself when the pope teaches on faith and morals. that is self evident. however it contradicted itself on limbo, salvation of non catholics and the death penalty.
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limbo -
Popes of the Roman Catholic Church have taken four contrary positions regarding the fate of infants who die without baptism. The lot assigned by popes to the infants has gradually changed from including hell fire, through involving the pain of loss only and then no pain at all, to full beatitude in heaven.
The new Catechism, published by John Paul in 1992, encourages us to hope that unbaptized infants go to heaven.
“As regards children who have died without baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus’ tenderness toward children which caused him to say: “Let the children come to me, do not hinder them,” allow us to hope that there is a way of salvation for children who have died without baptism.”
Answer: The Council of Florence stated the following about hell: The souls of those who depart this life in actual mortal sin, or in original sin alone, go down straight away to hell to be punished, but with unequal pains (Session 6 — July 6,1439).
The teaching of Carthage was infallibly approved as a rule of the Faith by Pope Zosimus and Pope Innocent I and by the ecumenical councils, which were approved by other popes.
“It has been decided likewise that if anyone says that for this reason the Lord said: “In my house there are many mansions”: that it might be understood that in the kingdom of heaven there will be some middle place or some place anywhere where happy infants live who departed from this life without baptism, without which they cannot enter into the kingdom of heaven, which is life eternal, let him be anathema. For when the Lord says: “Unless a man be born again of water and the Holy Ghost, he shall not enter into the kingdom of God” [John 3:5], what Catholic will doubt that he will be a partner of the devil who has not deserved to be a coheir of Christ? For he who lacks the right part will without doubt run into the left [cf. Matt. 25:41,46].”
“Babies dead without baptism go to Limbo, where they do not enjoy God, but neither do they suffer, because, having original sin alone, they do not deserve paradise, but neither do they merit hell or purgatory.” ~1905 Catechism of the Catholic Church
The XVI Council of Carthage (418) condemned the Pelagian fable that there is some place anywhere where infants who died without baptism live in happiness (Limbo).
The Council taught the Catholic doctrine that infants go into the fire to be eternally punished with the devil, being on the left hand at the judgement.
Pope Gregory the Great (-604) taught the eternal torment of infants in his Moralia on the Book of Job.
Gregory the Great: “For there be some that are withdrawn from the present light, before they attain to shew forth the good or evil deserts of an active life. And whereas the Sacraments of salvation do not free them from the sin of their birth, at the same time that here they never did aright by their own act; there they are brought to torment. And these have one wound, viz. to be born in corruption, and another, to die in the flesh. But forasmuch as after death there also follows, death eternal, by a secret and righteous judgment ‘wounds are multiplied to them without cause.’ For they even receive everlasting torments, who never sinned by their own will. And hence it is written, Even the infant of a single day is not pure in His sight upon earth. Hence ‘Truth’ says by His own lips, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. Hence Paul says, We were by nature the children of wrath even as others. He then that adding nothing of his own is mined by the guilt of birth alone, how stands it with such an one at the last account, as far as the calculation of human sense goes, but that he is ‘wounded without cause?’ And yet in the strict account of God it is but just that the stock of mortality, like an unfruitful tree, should preserve in the branches that bitterness which it drew from the root. Therefore he says, For He shall break me with a tempest, and multiply my wounds without cause. As if reviewing the woes of mankind he said in plain words; ‘With what sort of visitation does the strict Judge mercilessly slay those, whom the guilt of their own deeds condemns, if He smites for all eternity even those, whom the guilt of deliberate choice does not impeach?’” (Moralia 9: 32)
According to Pope Innocent, infants suffer the pain of knowing that they have lost the vision of God but they do not have the pain of fire.
“Pope Innocent’s teaching is to the effect that those dying with only original sin on their souls will suffer ‘no other pain, whether from material fire or from the worm of conscience, except the pain of being deprived forever of the vision of God.’ It should be noted, however, that this poena damni incurred for original sin implied, with Abelard and most of the early Scholastics, a certain degree of spiritual torment.” (Toner, Catholic Encyclopedia 1910, Limbo)
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salvation of non catholics
noncatholics aren't saved versus they could be saved. the 'strict' teachings are bolstered by the fact that the laity and people in the church understood that no one could possibly be saved as a noncatholic when those teachings came down.
The Catechism of the Catholic Church states that the phrase, "Outside the Church there is no salvation", means, if put in positive terms, that "all salvation comes from Christ the Head through the Church which is his Body", and it "is not aimed at those who, through no fault of their own, do not know Christ and his Church".[34] At the same time, it adds: "Although in ways known to himself God can lead those who, through no fault of their own, are ignorant of the Gospel to that faith without which it is impossible to please him, the Church still has the obligation and also the sacred right to evangelize all men".[35] The Catechism also states that the Catholic Church "is joined in many ways to the baptized who are honored by the name of Christian, but do not profess the Catholic faith in its entirety or have not preserved unity or communion under the successor of Peter", and that "those who have not yet received the Gospel are related to the People of God in various ways".[36]
Council of Florence, Cantate Domino (1441): "The most Holy Roman Church firmly believes, professes and preaches that none of those existing outside the Catholic Church, not only pagans, but also Jews and heretics and schismatics, can have a share in life eternal; but that they will go into the 'eternal fire which was prepared for the devil and his angels' (Matthew 25:41), unless before death they are joined with Her; and that so important is the unity of this ecclesiastical body that only those remaining within this unity can profit by the sacraments of the Church unto salvation, and they alone can receive an eternal recompense for their fasts, their almsgivings, their other works of Christian piety and the duties of a Christian soldier. No one, let his almsgiving be as great as it may, no one, even if he pour out his blood for the Name of Christ, can be saved, unless he remain within the bosom and the unity of the Catholic Church". The same council also ruled that those who die in original sin, but without mortal sin, will also find punishment in hell, but unequally: "But the souls of those who depart this life in actual mortal sin, or in original sin alone, go down straightaway to hell to be punished, but with unequal pains".[26]
Fourth Lateran Council (1215): "There is but one universal Church of the faithful, outside which no one at all is saved".[25]
morality of the death penalty
death penalty is admissible and morals versus it is inadmissible and immoral
In 2018, the Catechism of the Catholic Church was revised to read that "in the light of the Gospel" the death penalty is "inadmissible because it is an attack on the inviolability and dignity of the person" and that the Catholic Church "works with determination for its abolition worldwide."[3][4][5]
Pope Innocent I in his letter Ad Exsuperium, Episcopum Tolosanum (PL 20, 495) defended the death penalty:[11]
It must be remembered that power was granted by God, and to avenge crime the sword was permitted; he who carries out this vengeance is God's minister [Romans 13:1–4]. What motive have we for condemning a practice that all hold to be permitted by God? We uphold, therefore, what has been observed until now, in order not to alter the discipline and so that we may not appear to act contrary to God's authority.