The wrath of God is being revealed from heaven against all the godlessness and wickedness of men who suppress the truth by their wickedness, since what may be known about God is plain to them, because God has made it plain to them. For since the creation of the world God’s invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse. Romans 1:18-20

This passage in Romans tells us that God’s eternal power and divine nature are clearly visible in His creation, other believers and His Bible. All people know there is a God. Some accept it and some reject it.

The expression “having God in your heart” means that you have a relationship with Him. The Bible extends that expression to more than just a Valentine feeling of love, but a whole body surrender to the love of God.

Love the LORD your God with all your heart and with all your soul and with all your strength. Deuteronomy 6:5

So God is not in everyone’s heart in terms of having that love relationship with Him. God did create man, so we are naturally wired to believe in Him and recognize Him in the world around us.

However, God is in the hearts of bad people, since the definition of “bad” is sinful. All believers continue to sin, making them bad, however they are forgiven, constantly striving to keep that relationship alive. So a “bad” person who has a relationship with God can have God in his heart. But a “bad” person who refuses a relationship with God would not Him in his heart.

Their heart is too filled with themselves and there is no room for God.