וּבְנֵ֣י הַנֵּכָ֗ר הַנִּלְוִ֤ים עַל־יְהוָה֙ לְשָׁ֣רְת֔וֹ וּֽלְאַהֲבָה֙ אֶת־שֵׁ֣ם יְהוָ֔ה לִהְי֥וֹת ל֖וֹ לַעֲבָדִ֑ים כָּל־שֹׁמֵ֤ר שַׁבָּת֙ מֵֽחַלְּל֔וֹ וּמַחֲזִיקִ֖ים בִּבְרִיתִֽי׃
Also the aliens, that join themselves to the LORD, to minister unto Him, And to love the name of the LORD, To be His servants, Every one that keepeth the sabbath from profaning it, And holdeth fast by My covenant:
Read kabbalistic (mystical) interpretation of Isaiah 56:6: and classical mystical sources.