Charles Ellicott 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 ... —Proselytes also were to share in the blessings of the wider covenant. The words “to serve him” have been interpreted by some as referring to menial functions, like those of the Nethinim, hewers of wood and drawers of water (Joshua 9:27; Ezra 8:20). The usage of the word, however, limits it to honourable functions.

The germ of Isaiah’s thought appears in Solomon’s dedication prayer (1 Kings 8:41–43). It receives its highest development (in its entire separation from the building with which this idea is associated, both in Solomon's prayer and in this prophecy) in John 4:23. Compare a further emancipation from the bondage of the law in Isaiah 66:21.