John Calvin Commentary Psalms 136:10

"To him that smote Egypt in their first-born; For his lovingkindness [endureth] for ever;" — Psalms 136:10 (ASV)

Who smote the Egyptians in their first-born; some read with their first-born, but the other rendering reads better. As we do not intend to sermonize on the passage, it is unnecessary to detain the reader here with many words, as nothing is mentioned other than what has been treated elsewhere.

We may note that the Egyptians are rightly said to have been struck in their first-born. Although the other plagues occasionally terrified them, they continued in their outrageous obstinacy; however, they were broken, subdued, and submitted because of this last plague.

Since it was not intended to recount all the wonders performed in succession in Egypt, the whole is summarized in the statement that He led His people out from its midst with a mighty and a stretched out arm. For, being hard-pressed on every side, it was only by a wonderful display of divine power that they could make their escape.

The figure of an outstretched arm is appropriate, for we stretch out our arm when a great effort is required. This implies that God exerted an extraordinary, and not a common or slight, display of His power in redeeming His people.