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"Colossians 1:15
“He is the image of the invisible God, the firstborn over all creation.”
The Watchtower teaches that the word “firstborn” in the above verse means that there was a point in time when Jesus was born or came into existence.
Response: Although the word “firstborn” can mean one literally born first in time, it actually carries a much more important meaning. Firstborn can also mean “preeminent one” or “first in rank”. In the Jewish culture, the firstborn son had special rights and privileges as the heir or the one who received a double portion of the father’s estate. By reading the surrounding verses in Colossians chapter 1, we can see what the author intended firstborn to mean. While Paul magnifies the Lord Jesus as the Creator and Sustainer of all things, in verse 18 he says, “that in all things He may have the preeminence”.
Point out to your Jehovah’s Witness friend the difference between the words “firstborn” and “first-created”. The New Testament uses the word firstborn approximately 7 times in reference to Christ, but not once does it refer to Christ as first-created. In fact, they are two entirely different words in the original Greek language.
Two ways you can illustrate the meaning of the word firstborn are:
1. Compare Genesis 41:51-52 with Jeremiah 31:9. Notice the change. Although Man***eh is the first one born to Joseph, Ephraim becomes the firstborn, or preeminent one.
2. In Psalm 89:27 David is called the firstborn, even though he was the last one born to his father, Jesse."
the above is taken from the following link
http://www.towertotruth.net/AnsweringJWArguments.htm