The adverse influences and tendencies which continue after the new birth imperil the very existence of the new principle of love to God by overcoming and choking it, unless it is continually nourished and strengthened by divine grace. Strength is supplied to the believer by the inner presence of the Holy Spirit. His indwelling is by faith. If faith declines, the Spirit's sphere in the soul is narrowed. If confidence in God is "cast away" — a possible act against which we are warned in the Scriptures (Heb. x. 35) — then the Spirit withdraws, or rather, is excluded by unbelief, and love, the vital spark of the spiritual life, expires. Hence the question whether the Spirit shall be a merely transient impulse toward purity, or a lasting power, depends on the free will of the regenerate soul. The parable of the sower is exemplified to-day in the case of those who have no depth of earth. Their love to Christ soon degenerates into a mere sentiment with little or no influence on practical life, and in a short time the sentiment itself entirely evaporates, and the soul becomes "twice dead, plucked up by the roots" (Jude 12 ). What is the safeguard against such a disaster? It is such an indwelling of the fulness of the Spirit as excludes everything contrary to the divine nature by filling and flooding the soul with a love that is ever enlarging the vessel and ever filling it to the brim. Then love is perfect in the sense that it is no longer mixed in kind and so weak in degree as to be unable to encounter the temptation successfully. Says Prof. Candlish:
The new life of Christianity is a unity, and though, on account of the imperfect and abnormal condition of most Christians, it does not show itself with perfect symmetry, yet it tends toward moral excellence and perfection in every direction, and the more vigorous the central principle of religious life is, the more will particular virtues be developed and increased.
— edited from The Gospel of the Comforter Chapter 14.