Hebrews 13:4

Christians also need to maintain a high regard for marriage and to remain sexually pure. God’s judgment will follow the sexually impure (cf. Hebrews 12:29). Under the Old Covenant the Israelites were to punish fornicators and adulterers, but under the New Covenant God does it.

"How does God judge fornicators and adulterers? Sometimes they are judged in their own bodies (Romans 1:24-27). Certainly they will be judged at the final judgment (Revelation 21:8; Revelation 22:15). Believers who commit these sins certainly may be forgiven, but they will lose rewards in heaven (Ephesians 5:5 ff). David was forgiven, but he suffered the consequences of his adultery for years to come; and he suffered in the hardest way: through his own children." [Note: Wiersbe, 2:327.]