The Psalmist is the speaker in all the passages the Hebrews writer quotes between verse 6 and 12.

The devil is the father of lies and he has been hard at work with his lies. Little children, do not be deceived by his schemes which he accomplishes through his servants.

Take care, brethren, that there not be in any one of you an evil, unbelieving heart that falls away from the Living God. But encourage one another day after day, as long as it is still called “Today,” so that none of you will be hardened by the deceitfulness of sin. For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end. Hebrews 3:1-14