Notice the context here: "My salvation is near." This is an end-time prophecy, for our day and age. What He is talking about here actually began with the ministry of Jesus Christ. We have not reached its peak by any means, but it began when Jesus Christ began to turn to the Gentiles. It was a signal that He was moving away from Israel, and something new was beginning.
The "foreigners" here are Gentiles who are becoming part of the church of God. God was dealing with Israel primarily, and certainly Gentiles and foreigners were permitted to be a part of Israel. But this is signaling something that is far bigger than that! This is signaling something that involves worldwide activities.
Eunuchs were usually set apart to serve the king, and were castrated. Another way of saying it is that they were mutilated. But every one of us are mutilated spiritually. And we have been set apart to serve the great King—God. This really fits everybody in the church, not just Gentiles!
God is very concerned about the way things are done. Twice here He uses the word "defile." "Do not defile My Sabbath." In verse 2 and then again in verse 6, He says that those who "do not defile" are the ones who will be blessed. They will be His servants.
Can perfection come to any person who does not care how things are done? The answer is obvious. In regard to the Sabbath, it not only matters to God that we do it, but also how we do it. Why? Because it affects the outcome of the product! A person can produce something of poor quality, and they have, indeed, produced something. But if they produce something in which they really care about what the outcome will be, they are going to produce something that will be closer to perfection. It is like the difference between a mass-assembled automobile and a Rolls Royce built by hand—"customized" from the bottom up. Because a person cares about the product, he will produce something that is much better.
That is the principle involved here. God is concerned that "His people—the Gentiles" and "His people—the eunuchs" do not defile the Sabbath. Defile means to pollute, to make impure, unclean, dirty, corrupt, to profane. Biblically, it means "to put to common use."
A polluted steam is unfit for drinking. It might even be unfit for swimming. It might be so unfit that even fish cannot live in it. It might even be deadly. Think about that in relation to the Sabbath. Does it matter how something is done? God is concerned—otherwise something like this would not appear in His Word.
John W. Ritenbaugh
The Fourth Commandment (Part 4)
A sign identifies a business, a street, a product, etc., and so does the Sabbath! It identifies God's people. Notice also that this covenant, made after the ratification of the Old Covenant, bound the Sabbath as a "perpetual covenant" upon God's people.
Richard T. Ritenbaugh
It was Christ, as the God of the Old Testament, who actually created the Sabbath (John 1:1-3). It is a sign that identifies God's people just as a sign identifies a business or a street. Notice also that this covenant, made after the ratification of the Old Covenant, bound the Sabbath as a "perpetual covenant" upon God's people. Since the Sabbath has been in force from Creation, it is not just for the Jews, but for the foreigner and all mankind as well. All who keep the Sabbath properly are blessed.
Martin G. Collins
The Fourth Commandment