What does the Bible say about spiritual declension?

In Revelation 3:2, believers are called to 'be watchful' and to strengthen what remains in their spiritual life. This verse points to the concept of spiritual declension, wherein one's spiritual vitality may wane without a complete loss of grace. Just as the heart may continue to beat while weakened, a believer's faith can diminish in vigor and exercise even while retaining its essential character. The decline manifests through a lack of fervor and a waning resistance to sin, indicating an urgent need for introspection and renewal in the heart's affections toward Christ.