There are many arguments for and against the role illegal immigrants play in the economic and social structure of our country today. Most points made by one side are countered by polar opposite points from the other side. For instance, the faction that is adamantly opposed to tolerance of the illegal immigration population will say the II (let's use this shorthand to denote illegal immigrants) take jobs away from US citizens. The supporters of II say that the jobs they take are low wage positions that US citizens don't want and employers can't fill with US citizens. Both sides are probably partially right on this issue, but our purpose is not to explore the economics of the tolerance of II; it is to try to identify some Biblical principles concerning II. For most Christians, moral and ethical issues transcend economic and legal ones, every time.
First, the Bible makes no distinction between legal and illegal immigrants. In the days when the Mosaic Law was handed down, there were very few countries that levied restrictions on those who entered their borders. More people coming into your country generally meant a larger tax base and a larger manpower pool for agriculture, business, government, and military activities. So from now on in this essay we will take the example of the Old Testament and make no distinction between legal and illegal immigrants.
I was reared with the King James Version of the Bible, and most of the verses I memorized are from it; so it's easier for me to find pertinent passages in the KJV than in some of the newer "Plain English" versions. I also love the "poetic" language in the KJV, but that's just a personal preference. If I find a passage in the KJV that seems difficult to understand (usually because it uses some words that had different meanings during "King James'" day than it does in our time) I usually go to the NIV and read the passage to see if it is clearer in a more modern version.
Remember that the Mosaic Law was given to the Jews after they left Egypt. One of the first verses concerning immigrants (i.e., "stranger" or "sojourner") is found in Leviticus.
Leviticus 25:35-38: "And if thy brother be waxen poor, and fallen in decay with thee; then thou shalt relieve him: yea, though he be a stranger, or a sojourner; that he may live with thee. Take thou no usury of him, or increase: but fear thy God; that thy brother may live with thee. Thou shalt not give him thy money upon usury, nor lend him thy victuals for increase. I am the LORD your God, which brought you forth out of the land of Egypt, to give you the land of Canaan, and to be your God."