How can I know what is a sin and what isn't?

TL;DR:

Sin is anything—seen or unseen—that goes against God's will, character, or design, whether in our actions, thoughts, motives, or desires. We come to know what sin is through God's Word, the Spirit’s guidance, and a heart that seeks to love and honor God above all else.

what does the bible say?

Sin is anything we say, do, think, or feel that is against God. It is breaking God’s law, going against His character. Sin separates us from God and attempts to put us in His place. Adam and Eve sinned in the garden when they disobeyed God’s command and ate from the Tree of the Knowledge of Good and Evil. Their sin was not just about disobedience, as if sin is about not following rules. Rather, their sin revealed their distrust in God and trust in themselves. They sought their own desires instead of what God knew was best. Their sin was then passed down to every person both by nature and by choice. Sin is a threat that seeks to control us, but we are responsible for our response to it.

We can know what sin is by looking to God's Word, which defines sin not just as breaking rules but as anything that goes against the Lord’s character and will (1 John 3:4). Adam and Eve’s sin revealed that sin often starts in the heart before it shows up in actions (Genesis 3:6; Romans 1:21-22). Scripture also gives us clear examples of sin: the Ten Commandments outline moral boundaries (Exodus 20:1-17), while passages like Galatians 5:19-21, 1 Corinthians 6:9-10, and Proverbs 6:16-19 list specific behaviors God calls sinful. But sin is also more than just outward actions; it's failing to do what we know is right (James 4:17), acting without faith (Romans 14:23), or allowing anything—even something neutral—to control us and pull us away from God (1 Corinthians 6:12). As we grow in relationship with Christ, the Holy Spirit helps us discern sin more clearly, not only through biblical commandments but through convictions shaped by love for God and others, aiming to live in a way that reflects His holiness and grace.

from the old testament

The fall of humanity in Genesis 3 was ultimately the result of sin and disobedience. Pride, covetousness, the lust of the flesh, the pride of life, disobedience, distrusting God, a desire to be autonomous and make decisions apart from God are all deeply intertwined in the account of Adam and Eve’s choice to eat from the Tree of the Knowledge of Good and Evil. Their sin brought sin into the world.
Genesis 4:7 is the first time the word "sin" appears in the Bible. God warns Cain that sin is like a predator waiting to pounce. This verse shows that sin is a moral choice and a force that seeks to dominate us. It also implies personal responsibility—we are accountable for resisting sin.
Exodus 20:1-17 outlines God's moral law, known as the Ten Commandments. The Ten Commandments define sin by showing what God forbids: idolatry, murder, theft, adultery, lying, and coveting, among others. Breaking these commandments is sin (cf. Romans 7:7). These laws reflect God's holy character and His expectations for human behavior.
Isaiah 59:2 emphasizes the relational consequence of sin—it separates us from God: “But your iniquities have made a separation between you and your God, and your sins have hidden his face from you so that he does not hear.” Sin is a legal issue where we have violated God’s laws, but it also breaks fellowship with our Creator.
The Bible calls out many sins by name. Proverbs 6:16-19 lists pride, lying, and murder, among others, as sin.

from the new testament

Romans 3:23 says: “For all have sinned and fall short of the glory of God.” This verse universalizes the problem of sin. Every human being has sinned. Sin is not just breaking rules; it is falling short of God's perfect standard of holiness.
Romans 14:23 means that if a person believes something may be wrong and does it anyway, even if the act itself is not inherently sinful, it becomes sin for that person because they are not acting in faith. Ultimately, sin includes knowingly violating one’s God-given convictions.
A sure sign that something is a sin is if it is not personally edifying. In 1 Corinthians 6:12 (NIV), Paul says, "'I have the right to do anything,' you say—but not everything is beneficial. 'I have the right to do anything'—but I will not be mastered by anything." He would not be "mastered"—that is, he would not allow a gray area to become more important than his relationship with God.
Galatians 5:19-21 lists sexual immorality, idolatry, witchcraft, hatred, and jealousy as sin.
First Corinthians 6:9-10 calls stealing, swindling, and drunkenness by name as sin.
James 4:17 shows us that sin is not only doing wrong but also failing to do what is right. This is often called a “sin of omission.” It highlights the moral responsibility we have when we are aware of God's will.
First John 3:4 is one of the clearest definitions of sin in the Bible. Sin is described as "lawlessness," meaning a rejection of God's moral law. It is not merely a mistake but a willful disregard for God's authority.

implications for today

When we wrestle with the question of what is and isn’t sin, we're not just trying to follow a rulebook; we're learning how to live in close relationship with God. The Bible gives us clear boundaries, exposing sinful behaviors, but it also teaches us that sin can be more subtle. It can show up in moments when we go against our conscience, fail to do what we know is right, or allow something—even something neutral or good—to pull our hearts away from God. The goal of recognizing sin for what it is isn’t just to avoid punishment but to live a life that honors and reflects the character of Christ. As we grow in our relationship with God, the Holy Spirit sharpens our awareness of what pleases Him and what hinders our walk.

We are not left alone in this. God’s Word, the Spirit’s guidance, and the example of Jesus all help us understand what sin is and how to avoid it. But our daily choices come down to love. Do our actions reflect love for God and others? Are we doing what builds up our faith and serves others well? If not, even if the act itself isn’t on a sin list, we should pause and ask if it is truly honoring God. As Romans 14:23 reminds us, anything not done in faith is sin. Our aim should be to live every part of our lives “to the Lord,” seeking what draws us closer to Him, strengthens our spirit, and brings glory to His name.

understand

Sin is not just breaking rules—it’s choosing self over God.
Sin is anything that goes against God's character, will, or design—whether in action, thought, motive, or desire—both by commission and omission.
We learn what sin is through the Bible, the Holy Spirit’s conviction, and a growing relationship with God that shapes our conscience.

reflect

How do you personally define sin, and is that definition shaped more by culture, conscience, or Scripture?
When was the last time reading the Bible helped you realize that something in your life was sinful?
What role does the Holy Spirit play in helping you recognize sin in your thoughts, motives, or actions?

engage

John Wesley's mother put it this way: "Whatever weakens your reasoning, impairs the tenderness of your conscience, obscures your sense of God, or takes away your relish for spiritual things, in short if anything increases the authority and the power of the flesh over the spirit, that to you becomes sin, however good it is in itself." What makes it hard to recognize certain sins, especially those that start in the heart or seem culturally acceptable?
How does knowing God’s character help us better recognize what sin truly is?
Once we recognize something as sin, what should our next step be—and how can we respond in a way that draws us closer to God rather than just feeling guilty?