But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
One another, х heautous (G1438)] - 'yourselves; let each exhort himself and his neighbour.
Daily, х kath' (G2596) hekasteen (G1538) heemeran (G2250)] - 'on each day.'
While it is called Today - while the "Today" of grace lasts (Luke 4:21): before the day of judgment at Christ's coming (Hebrews 10:25; Hebrews 10:37). Tomorrow is the day 'when idle men work and fools repent. Tomorrow is Satan's today; he cares not what good resolutions you form, if only you fix them for tomorrow.
Lest any of you. The "you" is emphatic, as distinguished from "your fathers" (Hebrews 3:9). 'That from among you no one (B Delta f. But 'Aleph (') A C H, Vulgate, "no one from among you") be hardened' (Hebrews 3:8).
Deceitfulness - causing you to 'err in your heart.'
Sin - unbelief.