John Calvin Commentary Isaiah 48:18

"Oh that thou hadst hearkened to my commandments! then had thy peace been as a river, and thy righteousness as the waves of the sea:" — Isaiah 48:18 (ASV)

O if you had hearkened to my commandments! As the people might complain of being carried into captivity, the Prophet, intending to address those murmurs, points out the cause: that they did not submit to the doctrine of salvation and did not allow themselves to derive any advantage from it. He undoubtedly alludes to the song of Moses, in which very nearly the same form of expression occurs: O that they were wise, and that they understood! (Deuteronomy 32:29). לוא (lu) denotes a wish, such as 'O if!' or 'Would that!'

Not only does the Lord reason earnestly with the Jews for having disregarded the advantage, or "profitableness" (Isaiah 48:17), that was offered to them, but like a father, he deplores the wretchedness of his children. For he takes no pleasure in our distresses and is not severe unless we compel him by our wickedness.

This is therefore a figurative appropriation of human affections, by which God shows compassion for the ruin of those who chose to perish of their own accord rather than to be saved. For he was ready to bestow blessings of every kind if we did not drive him away by our obstinacy.

Yet it would be foolish to attempt to penetrate his secret counsel and to inquire why he did not add the efficacy of the Spirit to the external word. For nothing is said here about his power; instead, there is only a reproof of human hard-heartedness, so that they may be rendered inexcusable. Certainly, whenever God invites us to himself, complete happiness is clearly laid before us in his word, which we wickedly reject.

Then would your peace have been as a river. The word peace, as we have previously explained, denotes all prosperous events. It is as if he had said, "The richest plenty of spiritual blessings would have flowed to you abundantly, and you would have had no occasion to dread any change, because the blessing of God upon believers is never dried up."

And your righteousness as the waves of the sea. We might explain righteousness, which he connects with peace, to mean what is expressed by the familiar phrase (ton bon droict) “your right.” But I prefer to understand by the word “Righteousness” a well-regulated commonwealth, in which everything is administered in a regular and orderly manner; as if he had said, “You would have had everything well conducted at home, and would have had plenty and abundance of all things.”

And he properly connects this condition with “peace”; for when government is overturned, everything goes wrong and is out of order, and it is utterly impossible for us to enjoy “peace” if there is no “righteousness”—that is, a just and equitable administration of affairs. If, therefore, we are desirous of “peace,” let us also wish to have that blessed condition on which the Lord bestows his blessing.

Here some commentators speculate about spiritual “righteousness” and the forgiveness of sins, but they wander far from the Prophet’s meaning, which is plain and obvious.