What a verse this is! Just look past the old-fashioned language and the wealth of terms. No modern version can exhibit the depth of these terms as well as the pedantic old-fashioned KJV translation here.
We are the heirs of promise along with Abraham, for the promise was made to us as well as to him, since, as his seed, we too are included in that covenant of grace. God wants us to enjoy our inheritance ahead of time and to count it as certain before we have actually entered fully into it. How in that case can salvation be taken away from those who have received it? God does not wish us to count something as certain which we could lose again. Nor would he have us base our certainty on our own constancy, but on his faithfulness. The promise is secure because he has guaranteed it. Once we grasp this, it gives us strength to face all our trials in this world and to overcome by faith. Faith feeds on the promise and strengthens itself by means of the promise.