What does it mean that you reap what you sow (Galatians 6:7)?

from the old testament

• In the world, there are natural consequences for specific actions. For example, sowing a seed naturally results in a plant of the same type growing. Thus, one reaps the fruit of whatever seed was planted. This is true of the spiritual realm. There are natural consequences for sin. While warning the Israelites to obey God, Moses said, “But if you will not do so, behold, you have sinned against the LORD, and be sure your sin will find you out” (Numbers 32:23). He meant that there are natural consequences for sin.
• The book of Proverbs contains many sayings illustrating the different outcomes of sowing sin or sowing righteousness. For example, we read, “Because they hated knowledge and did not choose the fear of the LORD, would have none of my counsel and despised all my reproof, therefore they shall eat the fruit of their way, and have their fill of their own devices. For the simple are killed by their turning away, and the complacency of fools destroys them; but whoever listens to me will dwell secure and will be at ease, without dread of disaster” (Proverbs 1:29–33). In this example, the fool is the one who doesn’t listen to God. They get the natural result of destruction and death. Conversely, those who do listen to God reap a secure life.
• Another example, found in Psalm 1, contrasts the ends of the different paths between the righteous and the wicked. It reads, “[the righteous man] is like a tree planted by streams of water that yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers. The wicked are not so, but are like chaff that the wind drives away” (Psalm 1:3–4).
• Thus, there are natural consequences for sin. Those consequences are not arbitrary but based on God’s justice. Thus, “The wicked earns deceptive wages, but one who sows righteousness gets a sure reward” (Proverbs 11:18; cf. Hosea 10:12–13).

from the new testament

• Galatians 6:7 reads, “Do not be deceived: God is not mocked, for whatever one sows, that will he also reap.” Speaking to believers about the potential for believers to sin (Galatians 6:1–10), he was saying that God knows their sin (is not mocked). Because He knows, there are natural consequences for sinning. Just like one can expect to reap an apple tree if he sows an apple seed, so one can anticipate consequences when he “sows to his flesh” (Galatians 6:8).
• Paul often uses “flesh” to refer to our sinful nature (Romans 7:18). Here he’s saying that sinning (sowing to the flesh) comes with the natural result of corruption (Galatians 6:8a). He was not saying that if a believer sins he or she will lose salvation, but that the natural principle is that sin leads to death. A believer who willfully sins acts like one headed to death, and other passages warn that ongoing sin may indicate they aren’t actually saved (1 John 3:9–10). Thus, Paul used this illustration to caution them to stop sinning and to remind them, “let us not grow weary of doing good, for in due season we will reap, if we do not give up” (Galatians 6:9).
• Conversely, if one demonstrates they have salvation by being “one who sows to the Spirit,” he or she “will from the Spirit reap eternal life” (Galatians 6:8b). Salvation is through the Holy Spirit, and fruit of the Spirit is evidence that He dwells in an individual (Galatians 5:22–23).

implications for today

Everything has consequences, both negative and positive. For example, if we don’t pay our taxes one year, we can be sure the IRS will catch up with us in the years to come. Conversely, if we spend time studying Scripture, we become more like Christ. Everything has consequences because God built the world with natural and predictable causes and effects.

This means that how we spend our time today has an impact on our future, with some of it reaching eternity. For the person who rejects God today and lives a life of self-gratification, he or she will one day awake to the horror of reaping what was sown: an eternity of God’s wrath. Even those who regularly do good deeds, if they have not trusted Jesus, will find that their sinful deeds sown in this life were not negated by their good ones. They, too, will awake to an eternity of sorrowful reaping.

In fact, since everyone is sinful (Romans 3:23), not a single person alive today will, by default, reap anything but judgment. This is why Jesus came—to pay the penalty for repentant sinners. It is through His life, death, and resurrection that our eternities can change. When we repent of our sin and trust in Jesus as Lord (Romans 10:9–10), God no longer counts us His enemy (Romans 5:10). Instead, He credits us with Jesus’ righteousness (Romans 4:5) and calls us His children (John 1:12). That one act, genuine repentance and belief in Jesus, means we reap eternal life with Jesus (John 3:16).

This does not mean that after we believe, we can keep sinning. Rather, like Paul was saying, we must keep an eye on ourselves because there are still consequences for sin. But with the Holy Spirit, believers are able to do the kind of good deeds that please God (Philippians 2:13).

In short, what is done today impacts our eternity (2 Corinthians 5:10).