Experience of the Truth
A true Christian knows what it means to cry out to God for mercy, in concern lest he be condemned. He has more than a theoretical knowledge. He trembles when he views his guiltiness, and is driven to seek a covering in the blood of Christ. He abhors himself and grieves because he perceives the wickedness of his heart. He has come to know experientially that what Scripture says about such things is true.
This is what the older Reformed writers called experimental religion. A term was used which today is more commonly associated with the natural sciences, where a method of probing and investigation leads to an understanding of reality. In religion it means that we not only read and confess what Scripture teaches about salvation, but also are enabled by the Holy Spirit in our own experience to prove, feel the power, and enter into the truth of those things.
In this connection, a member of the Westminster Assembly referred to the knowledge of foreign countries acquired from maps. But map knowledge cannot compare with actually visiting the country, climbing its mountains, swimming in its rivers, and walking the streets of its towns. There are those who have seen godliness in the map only, hearing about it from the Bible, but have never had experience of the thing itself. At one time the young Saul of Tarsus had great confidence that all was well with him, but eventually that gave way to an awareness that his religious performances were deficient in the sight of God, and that he was still under wrath.
It is needful that preaching expound not only what Christ did outside of us to accomplish our redemption, but also what he does by the Holy Spirit in our hearts to apply that redemption. Such preaching includes the delineation of those biblical marks by which our lives give evidence that we have been renewed through union with Christ. This serves both to undeceive those who indulge a misguided hope, and also to promote that well-grounded assurance of salvation which will strongly confirm and embolden those who love God.
At the Reformation, entire nations were publicly committed to the truth of the Christian faith. But reformers such as Calvin understood that despite the near universal acknowledgment of biblical doctrine, multitudes in their midst had neither experienced the new birth, nor found personal contrition for sin and fled to the Savior for deliverance. It is not enough that children be raised in the church or be educated in a biblical world view. The reformers proclaimed to their congregations how to come to Christ, fenced the Lord's table against any who were in scandalous sin, and warned professing Christians of the peril of eternal judgment if they are not in saving union with Christ.
John Owen: The Things of This World
David Black: The Deceitfulness of the Heart
Archibald Alexander: Vital Piety
Thomas Chalmers: Self-Examination
Sherman Isbell, from The Master's Trumpet, issue 5: Recovering
Experimental Religion (pdf)