Belief in the Old Testament precludes belief in the New Testament or Koran

I posted a similar debate a while back, but I unfortunately didn't have time to finish it. My premise is simple; if you believe in the Old Testament's divinity, ala traditional Christianity or Islam, and that what it says is true, (at least) superficially understandable, and flawless (in the sense a perfect God wrote it) then any subsequent additions that change the message of the text, like the New Testament and the Koran, are incompatible with the Old Testament, and because they acknowledge the veracity of it, invalidate themselves. I have two basic proofs: 1) When God introduced the system of commandments on Mount Sinai, he spoke to the entire Jewish nation of 1.2 million people [3]. This is described in Exodus [1], and more extensively in Deuteronomy [2]. This presumably was so no one could possibly claim that they were being fooled by a charlatan. Either way, it certainly had that effect and, more importantly, set up a standard for divine revelation. If God spoke to you (or your grandfather, who told you about it), then a new "prophet" contradicting what God said is regarded with suspicion, regardless of how impressive he seems. Namely, if God changed His mind, why wouldn't He tell you like last time? Essentially, God set it up so no mortal man (or being bearing the appearance of one) can change the original commandments. This argument also has the added advantage of being valid even if one claims the Bible is inaccurate, since claiming that 1.2 million people saw God would be pretty hard to do if you couldn't't produce them (or their millions of descendants) to back you up. This pretty much verifies this particular set of passages (and possibly the whole Pentateuch). 2) The Old Testament predicts the coming of Jesus, Mohammad, and the like. In fact, it says that great people rise, perform miracles and wonders, and give signs to prove they are messengers of God. It says they will amass great followings. And then it says that if they change or defy any of the commandments, you should stone them. In fact, in one of the best examples [4], the Bible specifically says that they will be able to do all these things because God wants to test your faith. In fact, it even states that if he is similar to Moses himself (whom the Bible says was the greatest man to ever live [5]), he might still say things God hadn't commanded [6] (like a reversal of the Law, which is illegal), thus establishing no one is so great that they can be considered a greater authority than the Pentateuch. So, strangely for an omnipotent being who knows the future, God pretty much painted Himself into a corner. By telling everyone not to believe the people He was going to send, He ensured that people like Jesus or Mohammad would be rejected by anyone educated in His work, as indeed happened. This is not the nature of any caring or just God, or, indeed, consistent with any of God's apparent goals. I eagerly await my opponent's response. [1] http://www.biblegateway.com... http://www.biblegateway.com... [2] http://www.biblegateway.com... [3] http://www.biblegateway.com... (multiply by two for women, and by two again for people outside the age bracket) [4] http://www.biblegateway.com... [5]http://www.biblegateway.com... [6]http://www.biblegateway.com...