What does Galatians 6:9 mean?

Paul has just asked the Galatian Christians to be convinced, once again, that trusting their flesh in this life will only lead to corruption. Reliance on our own power only leads to decay and death. That is true whether we trust human effort to save us by following the rituals and sacraments of the law, or by following our own selfish, sinful desires. Salvation comes only by "planting" the Spirit. Through faith in Christ for the forgiveness of our sin, God gives to us His own Spirit. Only that Spirit will deliver eternal life in the end (Galatians 6:6–8).

Now Paul urges the Galatians not to grow tired of doing good. Doing good is hard work, especially if someone begins to doubt whether it matters. Paul is urging the Galatians to keep living in a way that is consistent with what they believe. They are free people in Christ, and God's Spirit is with them. Eventually, the crop of eternal life will come up and they will see it for themselves.

In a more immediate sense, the good works the Galatians are doing in the power of the Spirit will also bear fruit. God will often allow His people to see how their investment of themselves to do good matters both in this life and in the life to come.