ANSWER: According to Dictionary.com, illegal aliens are people who live in a country, other than the one of their birth, without having obtained citizenship status from the government.
In order to find out how illegal aliens should and should not be treated, we need to understand how God in the Bible instructed ancient Israel to treat the "strangers" (non-Israelites or Gentiles) living with them. Some erroneously believe that Israel was a closed society composed only of the descendants of Jacob. This, however, is simply not true. Not only does God allow aliens (also known as sojourners) to live among his chosen people, he specifically commanded how they were to be treated.
Those born in Israel were forbidden by God to oppress strangers - especially since at one time they, too, were strangers in a strange land.
21. You shall neither vex a stranger nor oppress him, for you were strangers in the land of Egypt. (Exodus 22:21, Holy Bible - a Faithful Version throughout)
9. Also you shall not oppress a stranger, for you know the heart of a stranger since you were strangers in the land of Egypt (23:9)
The ancient Israelites were also commanded to apply the SAME laws used for citizens to aliens as well, "There shall be one law to the one born at home and to the stranger (alien) that dwells among you." (Exodus 12:49)
God, no doubt foreshadowing the obedience from the heart that would be required under the New Covenant (Matthew 7:12), also commanded in the Bible that foreigners were not to be mistreated. He states that they were to be loved as a person loved themselves (Leviticus 19:33 - 34).
Those foreign born who lived in Israel could worship the true God as fully as any native-born citizen. The Bible does says, however, that they were expected to obey God's Law and were subject to the same penalties for disobedience as those born in the land. If they did obey, they could experience all the blessings that came with obedience to God's way just like any Israelite citizen.
Strangers were required to keep the Sabbath (Exodus 20:10), observe the Feast of Unleavened Bread (Deuteronomy 16:14), bring the appropriate sacrifice for sins of ignorance (Numbers 15:27-29), observe the Day of Atonement (Leviticus 16:29), and obey all the commandments, statutes, and judgments of God's Law (Exodus 12:49). Like those born in the land, strangers could offer sacrifices and take part in the red heifer ceremony (Leviticus 17:8 - 9, Numbers 15:14 - 16, 19:10).
The primary differences between natural citizens and uncircumcised aliens were that the latter could not eat the Passover (unless they were circumcised - Exodus 12:43 - 48) and were not released (if bondservants) in the Year of Jubilee (Exodus 12:43, 45, Leviticus 25:45 - 46).
The treatment of strangers found in the Old Testament was carried over into New Testament times when God's blessings would be available to people of all nations. The apostles and elders who gathered for what is known as the "Jerusalem conference" (Acts 15) recognized this truth. They placed no greater responsibility upon Gentile believers than that expressed in the Law of Moses. For whatever reason, they found it necessary to name four specific obligations (Acts 15:20), but beyond that, they knew that any other instructions were unnecessary.
What does all this mean for us today, especially for those living in countries like the United States that are struggling to deal with a growing influx of people from other countries?
The Bible does say, without a doubt, that those living in a country should not mistreat "strangers" that live among them. However, just as sojourners in Israel were required to obey God's laws before receiving his blessings, the strangers among us should also be required to obey the laws of the land before receiving the same benefits and privileges that come to citizens. If God's commands in the Bible regarding illegal aliens were observed by governments, they would avoid many of the problems they currently have regarding immigration.