When God made his promise to Abraham, he backed it to the hilt, putting his own reputation on the line.
He said, "I promise that I'll bless you with everything I have - bless and bless and bless!"
Abraham stuck it out and got everything that had been promised to him.
When people make promises, they guarantee them by appeal to some authority above them so that if there is any question that they'll make good on the promise, the authority will back them up.
When God wanted to guarantee his promises, he gave his word, a rock-solid guarantee -
God can't break his word. And because his word cannot change, the promise is likewise unchangeable.
It's an unbreakable spiritual lifeline, reaching past all appearances right to the very presence of God
where Jesus, running on ahead of us, has taken up his permanent post as high priest for us, in the order of Melchizedek.