God knew what would happen, which I agree with..since He knows everything.
Knowing everything that might happen is not the same as knowing everything that will or would happen. God does know the former, or so He tells me, but He repeatedly tells me that He does not know what I or others will choose, and parts of the future that depend on those choices are indeterminate to Him. He knows everything that might happen, but which of the mights actually takes place, He has to wait for those to whom He gave free will to make choices to know. When I ask God what will be, He tells me a few things that He is controlling outside of the influence of any free will human choices. But He says that many things have not yet been decided. He will give probabilities to alternatives, sometimes.
Yes, many agree on the "main" or "important parts" to some degree, but the fact that disagreements exist on what God wants or means (in which case someone is wrong) makes it possible that the core beliefs in Christianity could also be wrong (or untrue).
needs to be examined in the light of the thread's main question, what would you expect if Satan were real as well? If he were real, he would produce the sorts of phonies and disagreements that one sees, so as to make unbelievers doubt that there was a way (the church) to resist the "gates of hell." After all, it was the unity of the believers, their love for one another, that was supposed to be the clincher in evangelism.
God's answer to this is simple: learn how to know what is true and what is not. Ask the question, "Oh yeah, who says?" Look closely at inspired, perservering, inspiring art. Learn the scientific method. Review history. Pray with a listening ear, and test what you hear to see if it might not really be Jehovah speaking. This is His bottom line, really. If you don't "buy the truth, and not sell it. Also wisdom, understanding, and knowledge." you miss God's boat. In the light of all these methods, all the conflict and disagreement fade away. You learn to spot hypocrisy, rationalization, wishful thinking. Facts such as Calvin's Switzerland having the longest societal run of peace and prosperity begin to stand out, meshing with a clear scriptural emphasis on "keeping His commandments."
Sometimes those who sense prophetically that they are hearing God tell them what scriptural commandments mean get together with others who disagree with them, "in the Lord's name." We would hope that then He indeed would show up as promised, and speak again, in a way that dispells disagreement. I have seen this happen. But it has to be prophetic. The debaters have to stop and pray together for God to speak, and have to report what they hear, not what they think. I've never been in such a gathering, that ended in disagreement. But, I've been in many where there was a presumption of different persons "speaking for God" and disagreeing. When challenged, though, to affirm that what they were saying was what had "proceeded from the mouth of God," the disagreeing contestants could not.
Of course, refusal to gather, to pray, or to confess that what is being said was what was heard (rhema), all constitute hypocrisy, works of the enemy. Plenty of that to work through.
As you say, if Yeshua and Satan are really out there, these sorts of prayer meetings ought to produce remarkable agreement. I have found it so. That they are rare, and poorly reported, is consistent with the Satan part of the picture.