Salvation is a unity--a single piece, and yet a flowing stream. I become a Christian once for all on the basis of the finished work of Christ through faith; that is justification. But the Christian life, sanctification, operates on the same basis, but moment by moment. There is the same base (Christ's work) and the same instrument (faith). The only difference is that one is once for all and the other is moment by moment.
The whole unity of Biblical teaching stands solid at this place. If we try to live the Christian life in our own strength, we will have sorrow; but if we live in this way, we will not only serve the Lord, but in the place of sorrow He will be our song. That is the difference. The how of the Christian life is the power of the crucified and risen Lord, through the agency of the indwelling Holy Spirit, by faith moment by moment.
--Francis Schaeffer,
True Spirituality; as quoted in Lane T. Dennis' introduction to
Letters of Francis A. Schaeffer: Spiritual Reality in the Personal Christian Life (Crossway, 1985), 8; emphases original
