Albert Barnes 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 - The foreigner who will become a proselyte to the true religion.

That has joined himself - That has embraced the true faith and become a worshipper of the true God. It is evidently implied here that there would be such proselytes, and that the true religion would be extended so as to include and embrace them. The idea is that they are to be admitted to the same privileges as those who had long been recognized as the people of God.

The Lord has utterly separated - Let him not consider himself to be an outcast, or cut off from the privileges of the people of God.

This language is used with reference to the opinion which prevailed among the Jews that the Gentiles were excluded from the privileges of the people of God, and it is designed to intimate that in the future all such barriers would be broken down.

Those who entered the church as proselytes from the pagan world were not to come in with any sense of inferiority regarding their rights among His people. Instead, they were to feel that all the barriers which had previously existed were now broken down, and that all people were on a level. There is to be no assumption of superiority of one nation or rank over another; there is to be no sense of inferiority of one class in reference to another.

Neither let the eunuch say - This class of men was usually set over the harems of the East (Esther 2:3, Esther 2:14–15, Esther 4:5), and they were also employed as high officers at court (Esther 1:10, Esther 1:12, Esther 1:15; Daniel 1:3; Acts 8:27). The word is sometimes used to denote a minister of court, a court officer in general (Genesis 37:6, Genesis 39:1). The Targum often renders the word by רבא rabbâ' — “a prince.”

Behold, I am a dry tree - A dry tree is an emblem of that which is barren, useless, unfruitful. By the law of Moses such persons could not be enrolled or numbered in the congregation of the Lord (Deuteronomy 23:2).

The sense here is that they would no longer be subjected to the religious and civil disabilities to which they had previously been. These external barriers to the full privileges among the people of God would be removed. All classes and ranks would be admitted to the same privileges; all would be on the same level .