VI. Heb 6:18that by two immutable things, in which it is impossible for God to lie, we might have strong consolation, who have fled for refuge to lay hold of the hope set before us.
Of course, God is speaking about only two immutable things and God is not one of them. The first immutable thing is the promise which he willed the beloved of Hebrews 6 would inherit. The second immutable thing was his oath.
Heb 6:17 Thus God, determining to show more abundantly to the heirs of promise the immutability of His counsel, (το αμεταθετον της βουλης αυτου) confirmed it by an oath,
This is not talking about the essence or knowledge or attributes of God. God is saying he is not lying about his promise to the beloved. Some classical theologians would argue he is referring to all of God’s counsel as being immutable. Luke 7:30 says:
30 But the Pharisees and lawyers rejected the will of God for themselves, not having been baptized by him.
The will of God for the Pharisees and the lawyers included their baptism but they would not be baptized and rejected the will (βουλη) of God. If the Pharisees can reject the will of God then God’s counsel is not immutable. It is within the free will of man.