The mere reading of a book of daily devotion will not prove you a child of God; if you pray in private, then you have a sincere religion; a little religion, if sincere, is better than mountains of pretense. Home piety is the best piety. Praying will make you leave off sinning, or sinning will make you leave off praying. Prayer in the heart proves the reality of conversion. A man may be sincere, but sincerely wrong. Paul was sincerely right. “Behold, he prayeth,” was the best argument that his religion was right. If any one should ask me for an epitome of the Christian religion, I should say it is in that one word—“prayer.” If I should be asked, “What will take in the whole Christian experience?” I should answer, “prayer.” A man must have been convinced of sin before he could pray; he must have had some hope that there was mercy for him before he could pray. In fact, all the Christian virtues are locked up in that word, prayer. Do but tell me you are a man of prayer, and I will reply at once, “Sir, I have no doubt of the reality, as well as the sincerity, of your religion.”
From the Metropolitan Tabernacle Pulpit, Volume 1, Sermon 16 by Charles Haddon Spurgeon