Paul is rebuking the Corinthians for living in sin, when they ought to have been living for God and sharing the Gospel since their neighbors did not know the Lord. Some of them were not truly saved either. When we live in sin and are observed by unbelievers this turns them away from wanting to know the Lord. The Gospel is the power to change lives (Romans 1:16), so we ought to be different from the world. If you were the only person someone knew who was a believer would they be drawn to God by your life or turned off because you are no different than the rest of the world? Let us make sure we are truly saved and live for HIM.
1 Corinthians 15:34 Become sober-minded as you ought, and stop sinning; for some have no knowledge of God. I speak this to your shame.