a person’s actions determine what will later happen to her, especially when the actions and results are bad
—Tony always gossips about his friends, so it was no surprise when they started telling stories about him. You reap what you sow.

In his letters, Paul uses the example of planting and harvesting to teach about godly living and about giving to others. He writes,

Do not be deceived. God will not be made a fool. For a person will reap what he sows [whatsoever a man soweth, that shall he also reap], because the person who sows to his own flesh will reap corruption from the flesh, but the one who sows to the Spirit will reap eternal life from the Spirit. So we must not grow weary in doing good, for in due time we will reap, if we do not give up. So then, whenever we have an opportunity, let us do good to all people, and especially to those who belong to the family of faith. (Galatians 6:7-10)

Paul’s message is this: The seeds that are planted determine what will be harvested. If people do things because of sinful desires, the result will be death. If they do things to please God, the result will be life.

A few verses later, at the end of the letter, Paul returns to the subject of circumcision, saying that those who are trying to convince the Gentile Christians to be circumcised take pride only in outward obedience. He writes,

For those who are circumcised do not obey the law themselves, but they want you to be circumcised so that they can boast about your flesh. But may I never boast [God forbid that I should glory] except in the cross of our Lord Jesus Christ, through which the world has been crucified to me, and I to the world. (Galatians 6:13,14)

Here God forbid means “may God not let it happen.” Today, some still use the phrase with the same meaning, but usually it simply means “I hope it never happens,” with no actual thought of God. Therefore, it has much the same meaning as far be it from (me). And just as with that phrase, God forbid is often used sarcastically.