Romans 3:11

there is no one who understands,

no one who seeks God.

(v11, NIV)

This verse continues the quote from Psalms 14. Not only is there no one righteous, but no one understands or seeks for God. This is what it means to be under sin. When we are under sin, we do not understand or seek God. Our relationship with God is non-existent. We don't even seek after him. Our efforts and movements are in the opposite direction. Sin and being under sin is mainly an issue between you and God. It may affect your relations with other people; however, most people justify themselves by leaving God out of the picture. Many people look at others and compare. They will justify themselves, saying they're not as bad as someone else. However, God is left completely out of the picture. The issue isn't a comparison between you and other sinful people. Instead, the issue is between you and God. We sin, we are under sin and none of us is righteous. In addition, none of us understand or even seeks God. This is our condition and part of what it means to be under sin.