John Calvin Commentary Isaiah 54:4

"Fear not; for thou shalt not be ashamed: neither be thou confounded; for thou shalt not be put to shame: for thou shalt forget the shame of thy youth; and the reproach of thy widowhood shalt thou remember no more." — Isaiah 54:4 (ASV)

Fear not, for thou shalt not be ashamed. Here, as formerly, he strengthens the hearts of believers and addresses the whole Church, for the calamity was universal, and the Church appeared to be totally ruined. He tells her to be of good cheer and next assigns the reason: that the outcome of her troubles will be such that she “shall not be ashamed,” as if he had said, “Although for a time you are wretched, yet your affairs shall be prosperous.” And as it is elsewhere said, They who hope in the Lord shall not be ashamed (Psalms 25:3).

Blush not; that is, “cherish good hope, and be confident.” Those people “blush” who are ashamed and who, being disappointed in their hope, become disheartened. He next assigns the same reason: “for you shall not be exposed to shame.” I consider that here, as formerly, כי (ki) signifies for; and therefore, the same sentence is twice repeated under a variety of expressions, except that the former clause may relate to the disposition of the heart and the latter to the external cause. But the simpler meaning of both clauses is that it is a promise of success and prosperity, as if he had said that the calamity shall be brought to an end.

Yea, thou shalt forget the shame of thy youth. This is a confirmation of the former clause. He means the calamities which befell the Church while she was still young, and the remembrance of which will be wholly obliterated by the prosperity which she shall afterwards enjoy. We mentioned a little before that widowhood is a term used in regard to her because God had forsaken and, so to speak, had divorced her.