RobH_en_tn/heb/06/17.md

to the heirs of the promise

The people to whom God has made promises are spoken of as if they were to inherit property and wealth from a family member. Alternate translation: "to those who would receive what he promised" (See: rc://en/ta/man/translate/figs-metaphor)

the unchangeable quality of his purpose

"that his purpose would never change" or "that he would always do what he said he would do"