As with the bible, one must include and comprehend all church teachings in order to understand the whole. All men are born with sin, in the state of "original sin", a state of
injustice because it means that man is born alienated from His Creator, an unnatural, unjust, disordered state of being. And look around yourself and observe that all men are constantly looking for "life and happiness", often in very sinful ways. Understood correctly we're looking for God but in all the wrong places, idolizing lesser, created things over the Creator. But the desire for happiness is part of our created natures. As Aquinas taught, "God, alone, satisfies", but He's the last place where proud, fallen, man looks.
397 Man, tempted by the devil, let his trust in his Creator die in his heart and, abusing his freedom, disobeyed God's command. This is what man's first sin consisted of.278 All subsequent sin would be disobedience toward God and lack of trust in his goodness.
398 In that sin man preferred himself to God and by that very act scorned him. He chose himself over and against God, against the requirements of his creaturely status and therefore against his own good. Constituted in a state of holiness, man was destined to be fully "divinized" by God in glory. Seduced by the devil, he wanted to "be like God", but "without God, before God, and not in accordance with God".279
402 All men are implicated in Adam's sin, as St. Paul affirms: "By one man's disobedience many (that is, all men) were made sinners": "sin came into the world through one man and death through sin, and so death spread to all men because all men sinned."289 The Apostle contrasts the universality of sin and death with the universality of salvation in Christ. "Then as one man's trespass led to condemnation for all men, so one man's act of righteousness leads to acquittal and life for all men."290
Sacred Scripture and Sacred Tradition are two, equally important streams of revelation, BTW. Protestantism to one degree or another has thrown out much or most of the church's lived legacy as well as the unwritten traditions/teachings received at the beginning, aka "Tradition".
Mortal sin is "sin that leads to death". It must be persisted in and is always directly opposed to love of God and neighbor. This is why it can be said that "love fulfills the law" (Rom 13:10); love and sin are mutually exclusive. Grave matter also includes the deeds of the flesh outlined in Gal 5 and Rev 22, sins that demonstrate that one has turned totally against love and away from God-despite what they might
say, including any professions of "faith".
"Dear children, do not let anyone lead you astray. The one who does what is right is righteous, just as he is righteous. The one who does what is sinful is of the devil, because the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the devil’s work. No one who is born of God will continue to sin, because God’s seed remains in them; they cannot go on sinning, because they have been born of God. This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God’s child, nor is anyone who does not love their brother and sister." 1 John 3:7-10
1022 Each man receives his eternal retribution in his immortal soul at the very moment of his death, in a particular judgment that refers his life to Christ: either entrance into the blessedness of heaven-through a purification594 or immediately,595 -- or immediate and everlasting damnation.596
At the evening of life, we shall be judged on our love.597
1033 We cannot be united with God unless we freely choose to love him. But we cannot love God if we sin gravely against him, against our neighbor or against ourselves: "He who does not love remains in death. Anyone who hates his brother is a murderer, and you know that no murderer has eternal life abiding in him." Our Lord warns us that we shall be separated from him if we fail to meet the serious needs of the poor and the little ones who are his brethren. To die in mortal sin without repenting and accepting God's merciful love means remaining separated from him for ever by our own free choice. This state of definitive self-exclusion from communion with God and the blessed is called "hell."
1037 God predestines no one to go to hell;620 for this, a willful turning away from God (a mortal sin) is necessary, and persistence in it until the end. In the Eucharistic liturgy and in the daily prayers of her faithful, the Church implores the mercy of God, who does not want "any to perish, but all to come to repentance":621
1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.
Venial sin allows charity [love]to subsist, even though it offends and wounds it.
1856 Mortal sin, by attacking the vital principle within us - that is, charity [love]- necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation:
When the will sets itself upon something that is of its nature incompatible with the charity [love] that orients man toward his ultimate end, then the sin is mortal by its very object . . . whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery. . . . But when the sinner's will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial.130
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity [love]and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.