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. Gal. 6:7-8, NIV
Do not be deceived - First of all we see that it is possible for Christians to be deceived. We know from verse 1 that Paul is addressing Christians [Brothers]. Satan is the master deceiver. He deceived Eve who had super intelligence compared with us, who was in a perfect environment and who had no desire to displease God. If Eve could be deceived, then we are kidding ourselves if we think we cannot be. Only pride or ignorance would cause us to think otherwise.
The second thing we see is that, while it is possible to be deceived, it is also possible not to be deceived. If Paul instructs us not to be deceived, then it must be possible to see through the deceptions of the enemy. From the context, Paul is speaking of helping brothers ensnared in sin, being responsible for our own actions and receiving instruction in the Word of God. The best way, perhaps the only way, to avoid deception is to be in a community of believers who help each other, accept responsibility for their actions, and study the Word of God. The Word of God is Truth and Light. It alone can penetrate the dark lies of the enemy, but other believers balance and instruct us so that we do not misunderstand or misapply the Word of God.
God cannot be mocked - Although what is said above can be applied to any deception from the enemy, Paul has a specific deception in mind. Satan wants us to divorce actions and consequences. He wants us to think because we are Christians and forgiven by the blood of Christ, it doesn't matter if we sin or not. Having a careless attitude toward sin is attempting to mock God. This is not referring to the person who is struggling to conquer a sin or falls into a sin, but the person who uses salvation as an excuse to sin. Such a person is not only slapping God in the face, but also is despising the Cross of Christ. God cannot be mocked. While He may wait in mercy, He will not allow such actions to go on without punishment. God is a God of justice. Actions and consequences cannot be divorced.
A man reaps what he sows - When a farmer plants wheat seeds, he gets a crop of wheat. He would be foolish or ignorant to plant wheat seeds and then pray for a crop of barley. Yet we, as Christians, many times do the same thing. We plant selfishness, gossip, jealousy, anger, etc. and pray that when the crop comes up it will be all the Christian virtues. It doesn't work that way. We will reap what we sow only more of it and it will mature into something greater than what was planted. God will not hear our prayers to change the nature of our crop when we sowed it in careless or deliberate rebellion to His commands.
The one who sows to please his sinful nature, from that nature will reap destruction; - Since Paul is basically addressing Christians, he is not referring to salvation. He is attacking the idea that when a person is saved he can sin without consequences. Sin is a dangerous master. It always brings destruction. When we seek our pleasure in sinful activities, we will come under the judgment of God in this life. We will not know the joy of walking with God, the satisfaction of doing His will or reap the rewards He wants to give us in eternity. Instead, we will find the bitter fruit of a wasted life as well as whatever consequences follow the sin we have worshiped by giving it priority over God. It will not be a pretty sight. If we are genuinely Christian, we will make it to heaven but we will forever be shorn of whatever rewards and position we may have had.
the one who sows to please the Spirit, from the Spirit will reap eternal life - Sowing and reaping works both way. If we choose to do the things that please God we are sowing good seed, and we will reap the joy of the Lord and eternal benefits. Sometimes the sowing now is in tears and heartache, but when the time come to reap we will see a magnificent harvest - one that surpasses our greatest dreams. God always sees that the harvest is much more than the seed that was planted. His children may cry for a season now, but they will laugh for eternity.
There is a saying: "Play now, pay later. Pay now, play later." The "later" is always greater than the "now." People with an eye to the future are ready to make sacrifices now. People who can only think of the present will enjoy a short time of pleasure, but, in the long run, they will end up paying a heavy price for it…with interest.