Christian pray as one who is in a covenant of grace and not under a covenant of works. Joel and/or James Beeke gives this good reminder,
“Praying sincerely does not mean that we first attain a certain degree of sincerity and then pray. A sincere person knows and feels his imperfections; he deeply recognizes that his insincerity is not sincere enough. If we needed to become truly sincere in order for God to hear us, nobody would be able to pray. But someone asks, “Is not sincerity a quality that the Holy Spirit must work in my heart before I can truly pray?” We must answer “yes” and “no” to this question: Yes, the Holy Spirit must open our spiritual eyes to see sincerely that we are not sincere enough. But, no, we are never to think of our sincerity as a qualifying factor that earns us a right to God’s attention and answer. This would drag us back into a covenant of works, making us like pagans who think they must bring something in their hands to please their gods in order to be heard” (Developing a Healthy Prayer Life: 31 Meditations on Communing with God, 53).
Note: This also applies to our approach to public worship on Sunday when we don’t feel sincere enough to go to church. It even applies to pastors when they don’t feel sincere enough to preach. Remember, in Christ you are not in a covenant of works but a covenant of grace! (Romans 6:14; Galatians 5:18)