“And after he had dismissed the crowds, he went up on the mountain by himself to pray. When evening came, he was there alone.” -Matthew 14:23
The surest sign that you believe is that you pray more privately than publicly. If you can participate in lonely prayer–long and honest prayer, away from direct social influences–and still look yourself in the eye afterward, you have achieved the epitome of sincerity.
Why is private prayer such an important part of the Christian life? Because every Christian practice can break down into empty forms, but private prayer is the last to do so, and the most important in preventing others from doing so.
As long as we leave room for our faith to be insincere, it will be. The only way to trust, sincerely, is to start at the same place, every time: the heart. And that is what private prayer is: “the opening of the heart to God.”