Wherefore, as the Holy Ghost saith, Today if ye will hear his voice, harden not your hearts. The warning, thus led up to, is now introduced by a long quotation from Psalms 95:1-11., which is cited at length, because the writer is about to dwell on its whole significance in the remainder of this and also in the succeeding chapter.
The warning is connected by διὸ with the conclusion of Psalms 95:6. Since our continuing to be God's house is on the condition of our steadfastness, therefore beware of failing, as the Israelites referred to by the psalmist did.
With regard to the construction of the passage, there is some difficulty in discovering the apodosis to the initiatory καθὼς ("as saith the Holy Ghost"). It seems best to suppose one understood, being suggested by "harden not your hearts," which occurs m the midst of the quotation.
Sentences thus grammatically incomplete are in the style of St. Paul. Otherwise the apodosis must be found in βλέπετε (verse 12), the long intervening passage being parenthetical. It is, after all, only a question of grammatical construction; in any case the general meaning is clear.
As to the successive clauses of the quotation from Psalms 95:1-11.