John Calvin Commentary Isaiah 20:3

"And Jehovah said, Like as my servant Isaiah hath walked naked and barefoot three years for a sign and a wonder concerning Egypt and concerning Ethiopia;" — Isaiah 20:3 (ASV)

Three years. Why for such a period? Because that was the time granted to the Egyptians and Ethiopians, during which the Lord gave them a truce for repentance. At the same time, He wished to test the obedience of His people, so that they might relinquish unlawful aid without delay, and so that, though the Egyptians and Ethiopians appeared to be secure, His people might know that those nations were not far from ruin.

The Lord also intended to expose the rebellion of wicked men. For undoubtedly, many persons made an open display of their impiety when they despised the nakedness of the prophet.

The godly, on the other hand, moved by the sight of his nakedness, though the prosperity of the Ethiopians was delightfully attractive, still did not hesitate to fix their attention on the word.

What they were bound to consider was not the nakedness itself, but the mark which the Lord had put upon it. In the same manner, in the visible sacraments, we ought to behold those things which are invisible.