The Stranger

Numbers 3:10 YLT

10 `And Aaron and his sons thou dost appoint, and they have kept their priesthood, and the stranger who cometh near is put to death.'

A few verses before,  Aaron's oldest sons offered strange fire to the Lord and they died. Strangers are warned of the same fate. The Lord specified who could approach and how. 

The Hebrew word used here for stranger implies unfaithfulness, a tuning aside, of sleeping in another's bed. It is the same word used for the strange fire. 

Marriage is used throughout the Old Testament to speak of the relationship of God and His people. In the NT, the church is called the bride. That is why we are told He is a jealous God. 

זוּר * zûr * zoor * A primitive root; to turn aside (especially for lodging); hence to be a foreigner, strange, profane; specifically (active participle) to commit adultery: - (come from) another (man, place), fanner, go away, (e-) strange (-r, thing, woman)