“Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ.” -Romans 5:1
Whenever we see a wherefore or a therefore in Scripture, it’s a clue for us to go back and see what it’s there for. In our case, we have been following very closely with the developments of Paul’s argument. He has demonstrated, by appealing to the Covenant promise God made to Abraham which was imputed to him by faith, that justification is by faith and not works of the Law.
Now that we have established, says Paul, that justification is by faith in God and that our faith has secured the same for us that it did for Abraham, we can be sure that we have peace with God through our Lord Jesus Christ.
As the substance of God’s promise to Abraham was heirship (peace as a son) through the seed of a once dead and now resurrected son (of which Isaac is a type: first as being dead in Abraham’s loins and last as being resurrected on Mt. Moriah), so is the substance of God’s promise to us (peace as sons Cf. Romans 8:14-17) through God’s resurrected Son (Mt. Calvary).