"Thy word have I hid in mine heart, that I might not sin against thee." (Psalm:119:11)
The memorizing of Scripture has become almost a lost art among Christians today, and it is a great loss, for storing Scripture in the heart can be a great blessing in one's Christian life and witness.
The example of the early Christians is clear: Peter, Stephen, Paul, and others quoted extensively from the Old Testament in their impromptu sermons (Acts:2:17-21; 7:32-34; 13:33-35; etc.), and the same has been true of most of the great preachers and teachers of the Word ever since. There are multitudes of situations where it is of great help to be able to cite from memory an appropriate Scripture passage.
But the main reason for memorizing Scripture is the Word itself in effect commands it: "Let the word of Christ dwell in you richly in all wisdom; teaching and admonishing one another in psalms and hymns and spiritual songs, singing with grace in your hearts to the Lord" (Colossians:3:16).
The Lord Jesus Himself, in His humanity, no doubt learned the Scriptures as a boy (Luke:2:47, 52) and then quoted from them extensively in His many discourses, "leaving us an example, that ye should follow his steps" (1 Peter:2:21). He provided a perfect illustration of the validity of our text, defeating Satan in each of his three temptations simply by quoting a Scripture verse that He had long before hidden in His heart (Matthew:4:4, 7, 10). For the Word truly to be in our lives, it needs to be in our hearts.
--Henry M. Morris