Charles Ellicott Commentary Isaiah 56:3

"Neither let the foreigner, that hath joined himself to Jehovah, speak, saying, Jehovah will surely separate me from his people; neither let the eunuch say, Behold, I am a dry tree." — Isaiah 56:3 (ASV)

Neither let the son of the stranger ... — Two classes of persons were likely to suffer especially from Manasseh’s policy—(1) the heathen proselytes, who, as in Psalms 87, had been admitted as citizens of Zion under Hezekiah’s special protection; and (2) in the highest degree, those from that group who had been taken, as Ebed-Melech afterwards was (Jeremiah 38:7), into the king’s household as eunuchs. The courtiers of Manasseh would taunt them as aliens, and in the second case would press the letter of Deuteronomy 23:2.

The principle of Isaiah’s teaching was, of course, applicable to the Israelites who, like Daniel and his friends, had been mutilated against their will by heathen conquerors (Daniel 1:3), and most commentators refer the words to such cases. It is scarcely probable, however, that the household of Hezekiah would have been supplied with home-born eunuchs, and, on the hypothesis which I have adopted, I find in the eunuchs a sub-section of the proselytes. The words put into the mouths of the complainers are the natural utterances of men treated as they had been.