Jamieson, Fausset and Brown on Hebrews 3:13

13. one another--Greek, "yourselves"; let each exhort himself and his neighbor.

daily--Greek, "on each day," or "day by day."

while it is called To-day--while the "to-day" lasts (the day of grace, Lu 4:21, before the coming of the day of glory and judgment at Christ's coming, He 10:25, 37). To-morrow is the day when idle men work, and fools repent. To-morrow is Satan's to-day; he cares not what good resolutions you form, if only you fix them for to-morrow.

lest ... of you--The "you" is emphatic, as distinguished from "your fathers" (He 3:9). "That from among you no one (so the Greek order is in some of the oldest manuscripts) be hardened" (He 3:8).

deceitfulness--causing you to "err in your heart."

sin--unbelief.