"Sincerity" or being "sincere" is something a Christian must include in the qualities of the heart. According to the noted commentator Albert Barnes the word "sincere" means "that which is judged in the sunshine, in the brightest of light. It means literally - 'without wax.'"
When a person is sincere, his words and actions express purity and genuineness of heart. He is not ashamed to have his heart laid bare before both God and man. A sincere individual is one who does what he does in the light, without trying to hide anything from anyone. Furthermore, a sincere person doesn't have his heart chained and shackled with pride, prejudice, and make-believe. He's willing to admit when he's wrong, and he humbly asks for forgiveness and makes correction.
Paul closes the Ephesian letter with a blessing to the sincere. He says, "Grace be with all those who love our Lord Jesus Christ in sincerity. Amen." How about you? Are you sincere (without wax) in your love for Christ? May the Lord help us all to truly be sincere in our love and service to Him.