Your JW relative will likely say that God has always had an organization. He will point to examples such as Noah's day (you had to affiliate yourself with Noah in order to survive), the nation of Israel, and the First Century Christian Congregation.
You could respond with questions such as these:
- What organization did Job belong to?
- What was God's organization between A.D. 200 and the 1870s?
Another line of reasoning that JWs will use is: The Bible says the Good News has to be preached before the end comes. How else could the Good News be preached worldwide without an organization?
Note that this is a bit of circular reasoning--the Witnesses basically say that the Good News has to be preached, and the JW/WT organization is doing that preaching; thus there must be an organization and it must be the JW/WT faith. Now, in order to accept this line of reasoning, you must agree that we are in the "last days" as described by the Watchtower. If we're not in the "last days," then our situation is no different from folks in the 1400s when there was no true organization, according to the Witnesses.
So, I would approach it this way:
- Get the JW to admit that there was no "true Christian" organization in the 1400s (the WT teaches that this was the period of the wheat and the weeds where it was difficult to distinguish true/false Christians).
- Ask the JW why God didn't need an organization then, but needs one now? [The JW will likely say that it's because we're in the "last days" now and the Good News needs to be preached now.]
- Ask the JW something like this: "Ok, so you're saying that the reason God needs an organization is because we're in the last days? So, if it wasn't the last days, then God wouldn't necessarily need an organization, right? You admitted that he didn't need one during the Middle Ages, right?"
- Then--if you wish--go in for the kill: "So, how do you know it's the last days--it's because the WT organization says so, right? Isn't that circular reasoning? You say that God needs an organization because it's the last days, but the WT organization is the one telling you it's the last days? Doesn't that seem a bit sketchy to you?"
- The JW will likely stick to his guns on the last days issue, probably saying that the Bible is clear that these are the last days. This is fine. Use this as an opportunity to make him prove that the last days began in 1914. It's a great opportunity to poke holes in the 607 doctrine and highlight the WT's false prophecies and dubious history.