I confidently assert that the principal means by which most believers have become great in the Church of Jesus Christ is the habit of "diligent private prayer."
Look through the lives of the brightest and best of God's servants, whether in the Bible or not. See what is written of Moses, and David, and Daniel, and Paul. Note what is recorded about Luther and the Reformers. Observe what is related of the private devotions of Whitfield, and M'Cheyne. Tell me of one of all the godly fellowship of saints and martyrs, who has not had this mark most prominently--he was a "man of prayer." Oh, depend on it, prayer is power!
Prayer obtains fresh and continued outpourings of the Spirit. He alone begins the work of grace in a man's heart: He alone can carry it forward and make it prosper. But the Holy Spirit loves to be petitioned. And those who ask most, will always have most of His influence.
Prayer is the surest remedy against the devil and besetting sins. That sin will never stand firm which is heartily prayed against: the devil will never maintain influence over us when we ask the Lord to help us. But, then, we must spread out all our case before our Heavenly Physician, if He is to give us daily relief: we must ask Christ to send them back to the pit.
Do we wish to grow in grace and be very holy Christians? Then let us never forget the value of prayer.