In the book of Hebrews, the author issues a stern warning against the hardening of the heart, using the Greek term *sklerune* (Strong's 4645), which is the root of the medical term sclerosis. This term illustrates a spiritual condition where the heart becomes insensitive to God's voice, much like how physical sclerosis causes tissues or vessels to harden and lose function. The author references Psalm 95, highlighting Israel's 40-year rebellion in the wilderness as an example of such hardening, where despite witnessing God's deliverances, their hearts remained unresponsive to Him. This spiritual sclerosis is not about a lack of human empathy or emotion but pertains specifically to one's disposition toward God and His message. It develops gradually, often without noticeable symptoms, through the accumulation of worldly priorities, ideas, and habits that clog the heart, making it less receptive to divine truth. The author emphasizes that this hardening can lead to a refusal to hear Him who speaks, resulting in severe consequences, as seen in Israel's failure to enter the Promised Land due to unbelief and disobedience. Further, in Hebrews 3:13, the author identifies the deceitfulness of sin as a cause of this hardening, noting how sin promises fulfillment but delivers misery, subtly desensitizing individuals to God's standards over time. The urgency to guard against this condition is stressed as a daily effort, lest unbelief progresses unnoticed, akin to a silent buildup in the arteries. The author also points to environmental influences, such as societal norms and media, which can normalize sin and contribute to spiritual insensitivity if not countered by a continual return to God's word. In Hebrews 4, the narrative of Elijah in I Kings 19 serves as a cautionary tale of sclerosis, where his fixed worldview and self-focus clouded his perception of God's capabilities, leading to a form of unbelief that limited his usefulness to God. This example underscores the danger of allowing circumstances to harden one's heart, causing a departure from fully trusting and yielding to God. The author urges an immediate response to prevent such a condition, warning that neglecting this can result in missing out on the promises God offers, a fate that should evoke deep concern among those called to this great salvation.

Sin quietly produces hardness of heart, akin to a callus that forms over a break in a bone or stiffens a person's joints, paralyzing right action. Hardness is translated from *skleruno* (G4645), from which the name for multiple sclerosis is derived. In a moral context, it means impenetrable, insensitive, blind, or unteachable. A hardened attitude is not a sudden aberration but the product of a habitual state of mind that reveals itself in inflexibility of thinking and insensitivity of conscience. Eventually, it makes repentance impossible, and the will to do right is completely gone.