Take heed (literally, see), brethren, lest haply there should be (literally, shall be) in any one of you an evil heart of unbelief, in falling away from the living God. Here begins definitely the hortatory application of the warning of the ninety-fifth psalm.
Its drift, to the end of the chapter, is: You, being called under the SON to a far higher position than your fathers under Moses were, but the retention of your position being, as theirs was, conditional on your faithfulness, see that you do not forfeit it, as some of you may be in danger of doing.
That you may, if you are not careful, is shown by the very warning of the psalm, and by the example of your fathers, referred to in the psalm, all of whom, though called, failed of attainment through unbelief.
It is implied all along that the "today" of the psalm includes the present day of grace, and points to a truer rest than that of Canaan, still offered to the faithful. But the full bringing out of this thought is reserved for the next chapter.