World

"They are not of the world, just as I am not of the world." John 17:16

To be a Christian is to belong to the Kingdom of God and deny the kingdom of this world. You can't belong to both kingdoms simunteounesly, just like you can't serve two lords.

Sowing

"Don't be misled: No one makes a fool of God. What a person plants, he will harvest. The person who plants selfishness, ignoring the needs of others—ignoring God!—harvests a crop of weeds. All he'll have to show for his life is weeds! But the one who plants in response to God, letting God's Spirit do the growth work in him, harvests a crop of real life, eternal life." Galatians 6:7-8

To sow for your flesh is to prioritise vanities and worldly things. And so you reap what the world usually reaps - suffering. To sow for your Spirit is to prioritise the will of God and choose eternal life.