True believers may have the assurance of their salvation shaken, diminished, or temporarily lost in various ways:
as by negligence in preserving it,
by falling into some special sin which wounds the conscience and grieves the Spirit,
by some sudden or violent temptation,
or by God’s withdrawing the light of his countenance and allowing even those who reverence him to walk in darkness and have no light.
Yet, true believers are never completely deprived of that seed of God and life of faith,
that love for Christ and fellow believers,
that sincerity of heart and conscience concerning duty,
out of which—by the operation of the Spirit—this assurance may in due time be revived;
and by which, in the meantime, they are supported from utter despair.
(Westminster Confession of Faith 18.4 — Modern English Version)
Recommended: Silence by Jars of Clay