Assurance of faith is one of the great blessings of the Christian life. To be sure, it comes and goes, waxes and wanes. Sometimes the Christian is certain he or she is a beloved child of God. Other times the Christian doubts whether it is so. But assurance is something Christians should pray for, strive for, and be thankful when they have it. Charles Hodge has a good word on the grounds, or basis, for assurance in volume three of his Systematic Theology:
Many sincere believers are too introspective. They look too exclusively within, so that their hope is graduated (or grows – spl) by the degree of evidence of regeneration which they find in their own experience. This, except in rare cases, can never lead to the assurance of hope. We may examine our hearts with all the microscopic care prescribed by President Edwards in his work on “The Religious Affections,” and never be satisfied that we have eliminated every ground of misgiving and doubt.
The grounds of assurance are not so much within, as without us. They are, according to Scripture,
(1.) The universal and unconditional promise of God that those who come to Him in Christ, He will in no wise cast out; that whosoever will, may take of the water of life without money and without price. We are bound to be assured that God is faithful and will certainly save those who believe.
(2.) The infinite, immutable, and gratuitous love of God. In the first ten verses of the fifth chapter of the Epistle to the Romans, and in the eighth chapter of that epistle from the thirty-first verse to the end, the Apostle dwells on these characteristics of the love of God, as affording an immovable foundation of the believer’s hope.
(3.) The infinite merit of the satisfaction of Christ, and the prevalence of his continued intercession. Paul, in Romans 8:34, especially emphasizes these points.
(4.) The covenant of redemption in which it is promised that all given by the Father to the Son, shall come to Him, and that none of them shall be lost.
(5.) From the witness of the Spirit, Paul says, “We … rejoice in hope of the glory of God,” because the love of God is shed abroad in our hearts, by the Holy Ghost given unto us. That is, the Holy Ghost assures us that we are the objects of that love which he goes on to describe as infinite, immutable, and gratuitous. (Rom. 5:3–5.) And again, “The Spirit itself beareth witness with our spirit that we are the children of God.”
If, therefore, any true believer lacks the assurance of faith, the fault is in himself and not in the plan of salvation, or in the promises of God.
Charles Hodge, Systematic Theology, vol. 3 (Oak Harbor, WA: Logos Research Systems, Inc., 1997), 107.