Hebrews 4:1
1Let’s fear therefore, lest perhaps anyone of you should seem to have come short of a promise of entering into his rest.
Therefore—on the wilderness oath and unbelief of chapter 3—since a promise of entering his rest remains, let’s fear that any of us should seem to have come short of it. The promise is not revoked; the danger of missing it is named for the church. Wesley's whole chapter presses that a rest remains, and that falling short is no empty scare.
Both camps take let us fear seriously—the warning is no empty threat, not fake. The fork is not whether God meant the caution, but how far a hearer may fail to reach the rest. Fear here is holy urgency against presumption while the promise still stands.
Calvin: Let us therefore fear—not that which shakes the confidence of faith, but such as fills us with concern that we grow not torpid with indifference. Fear lest we be unfaithful to God's grace. No one comes short of the promise left us except he who by rejecting grace has first renounced the promise; by the fall of others we are taught humility and watchfulness.
The promise still stands, and let us fear is earnest means, not theater. Those who finally enter are kept; the fear God commands is the path of perseverance, not a wager that he may lose his own.
Key contrast Both refuse to treat let us fear or failed to reach it as sham while the promise stands. Arminians hear an actual danger that some who hear may seem—and in truth be—short of the rest; holy fear guards against failing to reach what grace still offers. Reformed interpreters hear the same urgency as God's means: the elect enter; fear guards against torpid unfaithfulness, and none finally come short except those who reject the grace they professed.