Sin

Is defined as, "Any thought, word, or deed against the Law of God." There two types of sin:

Mortal sin: A mortal sin is a transgression of the moral law in a serious matter, committed with clear advertence to the grievous nature of the act and with full deliberation and consent on the part of the will. It is called mortal since it deprives the soul of its supernatural life of sanctifying grace. It deserves eternal punishment (2 Thes 1:9), since the offence is a deliberate act of rebellion against the infinite majesty of God.

Venial sin: An offence against the law of God less grievous than mortal sin, not depriving the soul of sanctifying grace. A sin is venial either when the matter is not grave, or when, given grave matter, either full advertence to its gravity on the part of the intellect or full consent on the part of the will is wanting. Venial sins can be remitted by prayer or other good works.

(Definitions from A Catholic Dictionary, 1951)

References to Mortal Sin in Scripture:

"Know you not that the unjust shall not possess the kingdom of God? Do not err: neither fornicators, nor idolaters, nor adulterers, Nor the effeminate, nor liars with mankind, nor thieves, nor covetous, nor drunkards, nor railers, nor extortioners, shall possess the kingdom of God." 1 Corinthians 6:9-10

"Now the works of the flesh are manifest, which are fornication, uncleanness, immodesty, luxury, Idolatry, witchcrafts, enmities, contentions, emulations, wraths, quarrels, dissensions, sects, Envies, murders, drunkenness, revellings, and such like. Of the which I foretell you, as I have foretold to you, that they who do such things shall not obtain the kingdom of God." Galatians 5:19-21

"Mortify therefore your members which are upon the earth; fornication, uncleanness, lust, evil concupiscence, and covetousness, which is the service of idols. For which things the wrath of God cometh upon the children of unbelief" Colossians 3:5-6

"But the fearful, and unbelieving, and the abominable, and murderers, and whoremongers, and sorcerers, and idolaters, and all liars, they shall have their portion in the pool burning with fire and brimstone, which is the second death" Apocalypse 21:8

"But I say to you, that whosoever shall look on a woman to lust after her, hath already committed adultery with her in his heart" Matthew 5:28

References to Venial Sin in Scripture:

"For a just mall shall fall seven times and shall rise again: but the wicked shall fall down into evil." Proverbs 24:16

"But I say unto you, that every idle word that men shall speak, they shall render an account for it in the day of judgment." Matthew 12:36

Catholic Church Teaching on Mortal Sin:

"For whoever offends God, even by one mortal sin, instantly forfeits whatever merits he may have previously acquired through the sufferings and death of Christ, and is entirely shut out from the gate of heaven which, when already closed, was thrown open to all by the Redeemer's Passion." Catechism of Council of Trent, The Creed

"Returning now to the Sacrament, it is so much the special province of Penance to remit sins that it is impossible to obtain or even to hope for remission of sins by any other means; for it is written: Unless you do penance, you shall all likewise perish. These words were said by our Lord in reference to grievous and mortal sins, although at the same time lighter sins, which are called venial, also require some sort of penance. St. Augustine observes that the kind of penance which is daily performed in the Church for venial sins, would be absolutely useless, if venial sin could be remitted without penance." Catechism of Council of Trent, The Necessity of Sacrament of Penance.

We should, in the next place, carefully examine whether our consciences be defiled by mortal sin, which has to be repented of, in order that it may be blotted out before Communion by the remedy of contrition and confession. The Council of Trent has defined that no one conscious of mortal sin and having an opportunity of going to confession, however contrite he may deem himself, is to approach the Holy Eucharist until he has been purified by sacramental confession. Catechism of Council of Trent, The Eucharist

"Moreover we define that according to the general disposition of God, the souls of those who die in actual mortal sin go down into hell immediately (mox) after death and there suffer the pain of hell."
Encyclical "On the Beatific Vision of God" Issued by Pope Benedict XII in 1336

Catholic Church Teaching on Venial Sin:

"Unless you do penance, you shall all likewise perish. These words were said by our Lord in reference to grievous and mortal sins, although at the same time lighter sins, which are called venial, also require some sort of penance. St. Augustine observes that the kind of penance which is daily performed in the Church for venial sins, would be absolutely useless, if venial sin could be remitted without penance" Catechism of Council of Trent, The Sacraments - Penance

"Pastors should teach, first of all, that care must be exercised that confession be complete and entire. All mortal sins must be revealed to the priest. Venial sins, which do not separate us from the grace of God, and into which we frequently fall, although they may be usefully confessed, as the experience of the pious proves, may be omitted without sin, and expiated by a variety of other means" Catechism of Council of Trent, The Sacraments - Penance

"It cannot be doubted that by the Eucharist are remitted and pardoned lighter sins, commonly called venial. Whatever the soul has lost through the fire of passion, by falling into some slight offence, all this the Eucharist, cancelling those lesser faults, repairs, in the same way not to depart from the illustration already adduced as natural food gradually restores and repairs the daily waste caused by the force of the vital heat within us. Justly, therefore, has St. Ambrose said of this heavenly Sacrament: That daily bread is taken as a remedy for daily infirmity. But these things are to be understood of those sins for which no actual affection is retained." Catechism of Council of Trent, The Sacraments - The Eucharist

"The same result follows from the opinions of those who assert that little importance should be given to the frequent confession of venial sins. Far more important, they say, is that general confession which the Spouse of Christ, surrounded by her children in the Lord, makes each day by the mouth of the priest as he approaches the altar of God. As you well know, Venerable Brethren, it is true that venial sins may be expiated in many ways which are to be highly commended. But to ensure more rapid progress day by day in the path of virtue, We will that the pious practice of frequent confession, which was introduced into the Church by the inspiration of the Holy Spirit, should be earnestly advocated. By it genuine self-knowledge is increased, Christian humility grows, bad habits are corrected, spiritual neglect and tepidity are resisted, the conscience is purified, the will strengthened, a salutary self-control is attained, and grace is increased in virtue of the Sacrament itself. Let those, therefore, among the younger clergy who make light of or lessen esteem for frequent confession realize that what they are doing is alien to the Spirit of Christ and disastrous for the Mystical Body of our Savior" Encyclical "On the Mystical Body of Christ" by Pope Pius XII, 1943

Summary

It is clear from Scripture and past teaching of the Catholic Church, that even one mortal sin can keep us from heaven. Anyone who believes otherwise cannot say they are Catholic!