John Gill Commentary Isaiah 56:6

"Also the foreigners that join themselves to Jehovah, to minister unto him, and to love the name of Jehovah, to be his servants, every one that keepeth the sabbath from profaning it, and holdeth fast my covenant;" — Isaiah 56:6 (ASV)

Also the sons of the stranger that join themselves to the Lord, &c.] Having answered the objection and removed the discouragement of the eunuch, he now returns to the sons of the stranger, who also join themselves to the Lord, as the eunuch had done; see (Isaiah 56:4) and who do this, to serve him, and to love the name of the Lord, to be his servants; who give up themselves to him, not only to be saved by him, but to serve him in righteousness and holiness, with reverence and godly fear, and from a principle of love to him; being heartily desirous, and accounting it an honour, to be his servants: everyone that keepeth the sabbath from polluting it, and takes hold of my covenant; (See Gill on Isaiah 56:4).