'My Covenant' in the Bible

For thus says the LORD: Unto the eunuchs that keep my sabbaths, and choose the things that please me, and take hold of my covenant;

Also the sons of the foreigner, that join themselves to the LORD, to serve him, and to love the name of the LORD, to be his servants, everyone that keeps the sabbath from defiling it, and takes hold of my covenant;

As for me, this is my covenant with them, says the LORD; My spirit that is upon you, and my words which I have put in your mouth, shall not depart out of your mouth, nor out of the mouth of your descendants, nor out of the mouth of your descendant's descendants, says the LORD, from this time forth and forever.