Perfect example of a textual dispute! Read back a few verses to John 6:40 in both the NASB and KJV and I'll bet you'll be surprised.

Do you see what happened? A copyists read something similar and either accidently or intentionally added it again. Ever done that, copy the same thing twice? Mean to say one thing but think something else and say what your thinking? Sure you have. Its all understandable.
If you want to preach to someone the exclusivity of Christ all you need to do is read 7 verses back. When you do that the context of 6:47 which only says "believe" in the NASB fits perfectly. The reader will know what the author is talking about. The newer versions do not remove this. This is an awesome example of KJV only propaganda at work yet again.
Ah yes, I was waiting for this to be brought up. Its called the Comma Johanneum. Do you know the history of it? Let me fill you in. There was a man named Erasmus who was putting together a greek manuscript that is known as the
Textus Receptus (hereafter TR). The TR's first addition that came out in 1516 did not have the Comma in it. Basically, some folks were outraged, actually 2 fellas: Edward Lee, and Diego Lopez Zuniga who charged Erasmus with being an arian. Erasmus used about 6 byzantine mss to produce the TR, and in turn the KJV translators used the TR to produce the english version.
Erasmus stated basically "look guys, it is not in any of the manuscripts I have, but only in the vulgate." Then Erasmus said, "you show me a greek mss that has it in there and I'll put it in." So these two fellas went off and did just that. Its called Codex Montfortanius and is to this day repudiated to be a fabrication. But, Erasmus being trapped by his word threw it into the mix so as not to be called a liar. You can read his testimony to this in the annotations to the TR.
So, by the principal of cause and effect, we now have it in the KJV. By the way, the Byzantine MSS, which we have the most of, out of all of them, only like 5-6 even contain this verse. Plus, the ones that do have that reading have it off to the side in the margins, and not imbedded in the text itself, which indicates that its probably just a "help note" or a commentary of sorts. Thats a powerful testimony in and of itself that this was not part of John's original. It is safer to use the older ones that do not contain it. The exclusion of this verse in no way denys the principal of the trinity, which is most basic. Though it was nice of some pious copyist to throw it in the mix for us.
I think your intentions are good and no one here will argue that 1 John 5:7 is biblical whether or not it was part of the originals. But dont argue something and say look at what the original author wrote when there is a credible dispute about this. You could be lying with that claim and not even know it. Can you see what I am saying?
In conclusion I would like to say that constantly the NASB and ESV can continiously be shown to be superior to that of the KJV. It's not unsual since we have had 400 more years to find more manuscripts and advance the studies of the original languages and textual criticism. That doesnt mean that the KJV is inherently bad because I would argue otherwise, though I think it should be read for personal prefrence moreso than for the reason to believe its "perfect version."