And why must sincerity be one of the essentials of prayer which is accepted of God? Because sincerity carries the soul in all simplicity to open its heart to God, and to tell him the case plainly, without equivocation; to condemn itself plainly without dissembling [concealing of motives]; to cry to God heartily, without complimenting. … It knows not how to wear two masks, one for an appearance before men, and another for private use. It must have God, and be with him, in the duty of prayer. It is not a lip-labour that it regards, for sincerity, like God, looks at the heart, and that is where prayer comes from, if it be true prayer.
John Bunyan, Praying in the Spirit