I'd like to quote from a comment that "A Christian" posted several weeks ago that fits this topic pretty well. I don't have the direct link but I thought it was good enough to save on my own PC:
The men who run the Watchtower Society tell us that Jesus Christ appointed them over all his belongings, spiritually speaking, in the year 1919. As you know, they point to Christ's parable of "the faithful and discreet slave" found in Matthew 24:45-47 to support this teaching.
I suggest you ask your Dad a few simple questions. Questions such as these: How did the men who ran the Watchtower Society in 1919 find out that Christ had made this appointment? Did Christ tell the men who ran the Society in 1919 that he had done so, while appearing to them in a dream or vision? Did he send an angel to deliver this news to them? If he did not, how did they find out that Christ had appointed them to be his "Faithful and Discreet Slave" and thereafter act as his sole channel for distributing Bible truth on the earth? If Christ did not actually tell them, in some unmistakable way, that he had made such an appointment, then how has anyone ever really known that Christ actually ever really did such a thing? [M.J.'s comment: actually by their own admission in the "Proclaimers" book, they didn't even know they were "appointed" to be this "slave" until years after this "appointment", thinking that the "slave" was Russell until years after this incredibly momentous event!!!]
Jehovah's Witnesses usually answer such questions by saying something like this: "His appointment is obvious. After all, Jehovah's Witnesses understand the Bible far better than any other so-called Christian denomination. And no one else but us is preaching 'the good news of the kingdom' worldwide. And no other group has love among themselves like we do. So, we must be God's organization!"
However, even if all these things were true, would it really prove that Jesus Christ ever made such an appointment? After all, every Christian sect believes their group has the most truth, that their denomination is the one most approved by Jesus Christ, and that the leaders of their group are the most "faithful and discreet" slaves of Christ in all the earth.
If Jesus himself never actually told the men who ran the Watchtower Society in 1919 that he was then judging them to be "faithful", and that he was then appointing them "over all his belongings", was it right for them to tell the whole world that Christ himself had then done those things? Since Jesus Christ himself never made such an announcement to the men who ran the Watchtower Society in 1919, they had no way of then knowing for sure that Christ ever really made such an appointment. And, neither do the men who run the Watchtower Society today. And neither do any of Jehovah's Witnesses today.
The parable of the faithful and discreet slave informs us that when Christ returns he himself will judge which of his servants have faithfully taken good care of all he entrusted to them. With this in mind, is it the place of Christ's slaves to proclaim themselves to be faithful? No, it is not. Christ said he will judge his slaves. For a slave to judge himself as faithful and then tell all the world that Christ himself made that judgment is a very proud and presumptuous thing to do. And God assures us in His Word that, "I shall actually cause the pride of the presumptuous ones to cease." (Isaiah 13:11)