A man was filling out a job application when he came to the question "Have you ever been arrested?" He wrote, "No". The next question, intended for people who had answered "Yes" to the previous question, was "Why?" The applicant answered it anyway: "I never got caught". He evidently knew he was guilty of plenty!
So was the apostle Paul. He knew he had personally done wrong and sinned against God. He wrote, "I was formerly a blasphemer, a persecutor, and an insolent man" (1 Tim. 1:13). He even called himself the "chief" of sinners (v. 15).
We too were once separated from the Lord because of our sin and were considered His enemies (Rom. 5:10; Col. 1:21). But when we confessed our sin and acknowledged our need for His forgiveness, He cleansed us and made us new. Those of us who have known the Lord for many years may have the tendency to forget what we've been rescued from and forgive of. Sharing about our past and current failures and giving praise to God for forgiveness will help us not to come across as "holier-than-thou" to people who don't yet know the Lord. The truth is, we've all been guilty of plenty, and God deserves the glory for His mercy toward us. Grace is everything for those who deserve nothing.