In King David's psalm of repentance, he acknowledged his transgressions were against God.

For I know my transgressions, and my sin is always before me. Against you, you only, have I sinned and done what is evil in your sight, so that you are proved right when you speak and justified when you judge.
Psalm 51:3-4

David did hurt people with his sin directly, but his sins also gave a false image to people of the God who gave him his position as king.

Christians who behave badly also are in a position to either honor God or give others a reason to deny God's existence and goodness—presenting a false image of God.

Dear friends, I urge you, as aliens and strangers in the world, to abstain from sinful desires, which war against your soul. Live such good lives among the pagans [idol worshipers] that, though they accuse you of doing wrong, they may see your good deeds and glorify God on the day he visits us.
1 Peter 2:11-12

Our conduct in this life does affect how others see the God we profess to worship and whether or not they are willing to accept God and His Christ.