Christless religion, of whatever kind, says that we earn God's eternal approval by doing well.  But that is not the gospel of Jesus.  Paul explains by using the metaphor of wages - we earn what is agreed, we get what we deserve.  If we insist on earning divine payment for our good works, we will also deserve our punishment for everything bad.  That is no way to find peace with God.

Following the example of Abraham, who believed and obeyed what God said, and was credited as righteous even though he made some very bad mistakes, Paul refers to King David who was an adulterer and murderer.  After experiencing the misery of unconfessed sin, David discovered that repentance and faith in God's promises brought peace and joy.  David knew the blessing of being forgiven and restored into fellowship with God (Psalm 32:1-2).  His relationship with God was back on track.

The gospel message is illustrated by those two examples of trusting God.  Instead of paying us as our sins deserve (Psalm 103:10), God promises the free gift of eternal life to those who believe (John 3:16).  It has nothing to do with how hard we work at being good; it relies entirely on trusting God to give us what we do not deserve because that is what He has promised. That was how Abraham was justified (Romans 4:1-3).  King David too.  

We call this the 'grace of God'.  His delight in forgiving sinners who repent is the most precious gift to human beings.  We do not deserve His favour; we only deserve punishment.  But "the Lord has laid on Him [Jesus] the iniquity of us all" (Isaiah 53:6).  God gives us His righteousness in exchange for our wickedness: it is His gift to all who will receive it.  www.crosscheck.org.uk explains God's logic: it is the gospel message that your friends and colleagues need to see and hear - so give them the URL, watch it with them or offer to discuss it.  Initially it may not make sense by human logic but it is God’s only way to save us.  So, rejoice in God's free gift and tell others how to receive it too (Revelation 22:17).