". . . we speak as men approved by God to be entrusted with the gospel."
-- 1 Thessalonians 2:4
While most self-esteem is idolatry, I think many Christians are walking around joyless and timid because they don't realize the wonder of the gospel confidence in knowing that God approves of them in Christ. In Jesus, sin is forgiven, past is forgotten, religion is fulfilled, and eternity is secure.
If you believe in your heart and confess with your mouth that Jesus Christ is the Son of God and that he has been raised from the dead, God looks upon you with the approval he has for that Son. Do you know how fantastic that is? I almost didn't write it because it sounds so heretical! Jesus became your sin so that you could become his righteousness (2 Cor. 5:21). There is no more astounding fact in the history of mankind than that.
God may swipe the card of any of his children at any time, be they happy or sad, victorious or defeated, confident or depressed, and no matter how many charges we have made on it, no matter how many times we have tested debt with it, the report will return every time without fail: APPROVED.
“Christ’s righteousness is so imputed to believers that their justification is not merely the act of a sovereign dispensing with law but the act of a judge declaring the law to be satisfied.”
-- Charles Hodge