Adam Clarke on Hebrews 3:13

But exhort one another daily - This supposes a state of chose Church fellowship, without which they could not have had access to each other.

While it is called to-day - Use time while you have: it, for by and by there will be no more present time; all will be future; all will be eternity. Daily signifies time continued. To-day, all present time. Your fathers said: Let us make ourselves a captain, and return back unto Egypt, Num 14:4. Thus they exhorted each other to depart from the living God. Be ye warned by their example; let not that unbelieving heart be in you that was in them; exhort each other daily to cleave to the living God; lest, if ye, do not, ye, like them, may be hardened through the deceitfulness of sin.