Prepositions reveal location, time, and direction of their objects with the other elements of a sentence. Read the passage from Acts 8:38 below, and paint the picture in your mind of the characters and their locations (key prepositions are bold/underlined):
“And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.” (KJV)
“...and he commanded the chariot to stand still, and they both went down to the water, both Philip and the eunuch, and he baptized him…” (Young’s Literal Translation)
Is there a difference between getting into the water (baptism by immersion) and simply going to the water? Little prepositions surely affect doctrine and are not to be trifled with!
Back to unto. Because unto usually indicates the object has been delivered, personally accepted, completed, within (as compared to without), and close beside (not merely in the vicinity or direction of), it’s no surprise to find ‘close’ words almost always associated with unto. Nigh is one word, and cleave is another. Both words have a very close proximity and relationship sense.
There are exceptions. But these exceptions prove the rule:
“…father and his mother, and shall cleave unto his wife…” (Gen. 2:24).
“…father and mother, and shall cleave to his wife…” (Mt. 19:5).
So, again, we must ask, why does the initial passage use unto, while the referral, quoted of Jesus, uses the preposition to? My educated guess is that the context of Matthew 19 is about divorce: a man-made institution. However, the original, God’s plan without exceptions such as divorce, would be better and complete. This gets interesting and dovetails with certain verbs in Hebrews. These indicate that man’s work is not complete, not delivered; but, that God’s work is complete: “For every house house is builded by some man; but he that built all things is God.”
In conclusion, the use of to instead of unto suggests uncertainty, non-delivery. Divorce proposes the couple is no longer close, no longer cleaving unto each other. The use of unto would not be appropriate in this division context; however, it would be exactly correct when used in Genesis 2, which is absent of any “un-cleaving.”