While it is said, Today ... - That is, persevere as long as life lasts, or as long as it can be said “today;” and by persevering in this manner you will have evidence that you are the friends of the Redeemer. This is a quotation from Psalms 95:7. Paul means, undoubtedly, to make use of this language himself as a direct exhortation to the Christians to whom he was writing. He entreats them, therefore, as long as it could be said “today,” or as long as life lasted, to take care lest they should harden their hearts as had been done in the temptation in the wilderness.
Related Commentaries of Hebrews 3:15
Hebrews 3:15
15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.