Paul had been speaking about the Galatian believers' responsibilities to one another. He continued with these thoughts.
Galatians 6:6-10 (NIV)
Anyone who receives instruction in the word must share all good things with his instructor.
Do not be deceived: God cannot be mocked. A man reaps what he sows.
The one who sows to please his sinful nature, from that nature will reap destruction; the one who sows to please the Spirit, from the Spirit will reap eternal life.
Let us not become weary in doing good, for at the proper time we will reap a harvest if we do not give up.
Therefore, as we have opportunity, let us do good to all people, especially to those who belong to the family of believers.
In verse 6 Paul urged the believers, who have received instruction in the Word of God, to share all good things with their instructors. This concept falls in line with Paul's statement in 1 Corinthians.
1 Corinthians 9:14 (NIV)
In the same way, the Lord has commanded that those who preach the gospel should receive their living from the gospel.
Paul was concerned about the body of Messiah and how each one in the body should be cared for by the others. Those responsibilities included the welfare of their pastors and teachers.
In verse 7 Paul, again, anticipated some of the less than perfect responses that even the saints (forgiven sinners) can have towards others. No matter how hard someone may try, or think that God will not "notice", God cannot be mocked. He sees and knows everything. If someone believes that he or she can get away with treating others poorly he or she is deceived, because "a man reaps what he sows." There are always consequences to our actions.
Paul explained this more in verse 8. If someone acts according to his sinful nature, by being stingy, for example, he will reap from that nature. Often, stingy people receive stingy treatment in return. However, Paul's statement went further than this. He said that whoever sows from his sinful nature will reap destruction. He was talking about someone who allowed that sinful nature to have dominance in his or her life rather than by living in reliance on the Holy Spirit. In contrast, the one who acts according to the Spirit will reap eternal life.
Another issue that believers can experience is becoming weary in doing good (verse 9). Although sowing leads to reaping, the harvest is not always immediate. Many times our good actions are forgotten by others, or are met with evil in return. Yet, Paul assured the Galatians that even if they did not receive a return right away, eventually they would, at the very least, in eternity.
Therefore, Paul concluded that the believers were to continue in doing good deeds as the opportunities arose. They were to do good to all, but especially to the others in the body of Messiah.