What is the biblical basis for venial and mortal sins?

I do not have the knowledge to address specific Catholic doctrine concerning sin (even though I was raised Catholic), but your question did include “or lack support for” the Catholic belief.

Catholicism makes me think of the Pharisees and Sadducees. Some were well intentioned (Nicodemus -- John Ch 3 and Joseph of Arimathea – Luke 23:50) while others were promoting their own status (Matthew chapter 23 – Luke 20: 45-46). They got lost in the laws and rituals. Some of which were handed down by God, and others that were created by man (Isaiah 29:13)

So, I will reference the Bible concerning sin. Hopefully this will help in your quest for determining the validity of the Catholic view on sin.

I haven’t found anything in the Bible that talks about different types of sin. Meaning, for this sin you get a scolding….this one you get a spanking….. for this one you are sent to your room….and this one you will be grounded. It’s a silly analogy, but it’s trying to address the part of your question concerning sins that are “kinda bad”….”really bad”…..or “something worse”.

From what I can tell, there appear to be only two types of sin. Sins that can be forgiven…..and sins that cannot be forgiven.

• Matthew 12:31-32 and Mark 3:29 talk about how blaspheming the Holy Spirit is unforgivable

• Matthew 6:14-15 and Mark 11:25 talk about how you must forgive others, if not......God will not forgive your sins.

• Hebrews 6:4-6 talks about how it is impossible to bring back to repentance those thatwere once enlightened.

• Hebrews 10:26 says if you willfully sin, after receiving the knowledge of truth, there is no more sacrifice to cover you sins

Though I have noticed that the Bible does talk about different levels of reward and different levels of punishment.

• You shall be the least or greatest in heaven (Matthew 5:19 )
• They will be more severely punished (Mark 11:40)
• Some will be severely punished…..some will only be lightly punished (Luke 12:47-48)
  (Isaiah 28:22, Matthew 5:19, Matthew 11:22-24, Matthew 16:27, Mark 11:40, Luke 12:47-48, 1 Corinthians 3:11-15, 1 Timothy 5:17, James 3:1

If the above is accurate, then when we’re standing before Christ being judged for what we have done (2 Corinthians 5:10), our sins will either be forgiven or they won’t. Heaven or Hell.

If our sins are forgiven, and we enter heaven, then what determines the different levels of reward? I think there’s reason to believe that it’s the heart. In other words, our relationship with God.

Both the Old and New Testaments repeatedly talk about how God wants your heart.

• (Deuteronomy 13:3) The Lord your God is testing you to see if you truly love him with all your heart and soul.

• (1 Samuel 16:7) But the Lord said to Samuel, “Don’t judge by his appearance or height, for I have rejected him. The Lord doesn’t see things the way you see them. People judge by outward appearance, but the Lord looks at the heart.”

• (Isiah 13:9) And so the Lord says, “These people say they are mine. They honor me with their lips, but their hearts are far from me. And their worship of me is nothing but man-made rules learned by rote

• (Matthew 22:37-40) 37 Jesus replied, “‘You must love the Lord your God with all your heart, all your soul, and all your mind.’ 38 This is the first and greatest commandment. 39 A second is equally important: ‘Love your neighbor as yourself.’ 40 The entire law and all the demands of the prophets are based on these two commandments.”

• (Romans 2:29) No, a true Jew is one whose heart is right with God. And true circumcision is not merely obeying the letter of the law; rather, it is a change of heart produced by the Spirit. And a person with a changed heart seeks praise from God, not from people.

• (Hebrews 10:22) ...let us go right into the presence of God with sincere hearts fully trusting him.

  Look at King David. Look at the things he did in his life: coveted his neighbors wife, bore false witness against his neighbor, adultery, murder. Even on his death bed he was plotting revenge (1 Kings 2). But God called David a man after his own heart (1 Samuel 13:14; Acts 13:22)

  Look at Paul. His life’s mission was to wipe out the belief in Jesus Christ. He persecuted and killed Christians in an effort to accomplish this goal. Even after he surrendered his life to Christ, he talks about his struggle with sin. (Romans 7)

Sin separates us from God, but repentance draws us back to God.

• (Isaiah 59:2) But your iniquities have separated between you and your God, and your sins have hid his face from you, that he will not hear.

• (Psalm 51:17) The sacrifice you desire is a broken spirit. You will not reject a broken and repentant heart, O God

  Having grown up Catholic, there was the standing joke about partying all weekend then going to confession on Monday. This is not Catholic doctrine, nor is it the intent of the sacrament of reconciliation. But it is how many people live their lives. That’s not repentance: that’s an example of Isaiah 13:9

  And so the Lord says, “These people say they are mine. They honor me with their lips, but their hearts are far from me. And their worship of me is nothing but man-made rules learned by rote

Ironically, my New American Bible (a Catholic translation) has a footnote that gives a definition of the word repentance. "Repentance"- A change of heart toward God, reflected in the actions of one's own life

This probably falls more in line with what Paul meant when he said, "Can’t you see that his kindness is intended to turn you from your sin?" (Romans 2:4)

Jesus has provided an opportunity for grace and forgiveness. And if you accept this with a “repentant heart” you will go to heaven (God will not reject a repentant heart)

• Hebrews 10:19-22 19 And so, dear brothers and sisters, we can boldly enter heaven’s Most Holy Place because of the blood of Jesus. 20 By his death, Jesus opened a new and life-giving way through the curtain into the Most Holy Place. 21 And since we have a great High Priest who rules over God’s house, 22 let us go right into the presence of God with sincere hearts fully trusting him.

With this opportunity (the foundation of Jesus Christ) we are expected to build a relationship with the Lord (building materials), and this relationship will be judged by fire. The good news is that regardless of whether or not you struggled with that relationship, as long as you approached it with a repentant heart, you will go to Heaven. It may be like someone barely escaping through a wall of flames.

• (1 Corinthians 3:11-15) 11 For no one can lay any foundation other than the one we already have—Jesus Christ. 12 Anyone who builds on that foundation may use a variety of materials—gold, silver, jewels, wood, hay, or straw. 13 But on the judgment day, fire will reveal what kind of work each builder has done. The fire will show if a person’s work has any value. 14 If the work survives, that builder will receive a reward. 15 But if the work is burned up, the builder will suffer great loss. The builder will be saved, but like someone barely escaping through a wall of flames.

Or you may be called the least

• (Matthew 5:19) Whosoever therefore shall break one of these least commandments and shall teach men so, he shall be called the least in the Kingdom of Heaven; but whosoever shall do and teach them, the same shall be called great in the Kingdom of Heaven.

But you are in the Kingdom of heaven!

The alternative is to stand before the judgment of the Lord and hear this

• (Matthew 7:21-23) 21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven. 22 Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name and in your name drive out demons and in your name perform many miracles?’ 23 Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’

• (1 Corinthians 15:34) Think carefully about what is right, and stop sinning. For to your shame I say that some of you don’t know God at all.

The Bible doesn’t tell us if King David entered heaven as the least or the greatest or somewhere in between. It doesn’t tell us which of Paul’s building materials were burned by the fire of judgment. But it does tell us how much God wants us to be in relationship.

• (1 John 4:9-10) 9 God showed how much he loved us by sending his one and only Son into the world so that we might have eternal life through him. 10 This is real love—not that we loved God, but that he loved us and sent his Son as a sacrifice to take away our sins.

And it does tell us (repeatedly) that God wants your heart.

• (1 John 5:21) Dear children, keep away from anything that might take God’s place in your heart.

He knows that it’s broken and sinful. But he also knows if it’s repentant!

• (Ezekiel 18:30) “Therefore, I will judge each of you, O people of Israel, according to your actions, says the Sovereign LORD. Repent, and turn from your sins. Don’t let them destroy you! 31 Put all your rebellion behind you, and find yourselves a new heart and a new spirit. For why should you die, O people of Israel? 32 I don’t want you to die, says the Sovereign LORD. Turn back and live!

• (Jeremiah 17:10) But I, the LORD, search all hearts and examine secret motives. I give all people their due rewards, according to what their actions deserve.”