And therein lies the problem. God spoke to the Jews and told them what to expect, why would he "change it up" on them?
Keep in mind that the earliest Christians and Christian communities were primarily composed of Jews and understood themselves largely as Jewish in character--even when Gentiles started being added among their numbers.
It's not that God "changed it up", it's that there was--and still is--legitimate differences in interpretation of messianic expectation and messianic prophecy between Judaism and Christianity and these differences are in part one of the fundamental issues that led to a breech between the mainstream Jewish community and the fledgling Christian sect.
To a believer, "standard changes", "key cases", and "minor cases" may be something to shrug off and say "eh, it happens." To someone truly putting the Bible under a microscope and questioning God, these things all make the difference.
You would think that in a testament completely centered around one event, the resurrection, that the accounts of it would at least match. We don't find that to be the case, which really makes one lift an eyebrow.
The issue isn't to what degree the biblical texts are scrutinized, both believers and non-believers can scrutinize the text down to the nitty gritty, what makes a significant difference between the two is faith. Those textual discrepencies don't obliterate a Christian's faith because, at the end of the day, the essence of Christianity isn't the textual perfection of the Bible, but the centrality of the Jesus Event in the lives and faith of this living, breathing community called the Church.
I wouldn't quite call it a conspiracy, and even if so - conspiracies do happen. I don't know every facet of Christian history but it does seem Constantine would be politically motivated to spread Christianity.
He was politically motivated to use Christianity to unify the Empire. I have little doubt that his move to favor Christianity did have political motivations (whether or not he had true religious conviction isn't something I can say one way or another, that's between him and his Maker). However, it's excruciatingly important to point out that Constantine himself, apart from being the one to lead the move to pass the Edict of Toleration, and his active role in promoting Christianity as a favorable religion he did little to impact the substance of Christianity.
To offer an example of this: It's true that Constantine summoned the bishops to Nicea for a council to try and settle the Arian Controversy; but it wasn't the last council he summoned and ultimately Constantine had the Nicene-confessing bishops removed from their episcopal sees and saw Arius in favorable light; largely due to the influence of two close friends and court advisors, one of whom was an Arian sympathizer (Eusebius of Caesarea, the one who wrote the Church History and Life of Constantine) and one a devoted Arian and friend of Arius (Eusebius of Nicomedia, who baptized Constantine on his deathbed). Constantine's successors were often split, theologically, between the Nicene and Arian confessions, and these continued to wane back and forth until Emperor Julian, the last of Constantine's line, came to power and sought to restore and reinvigorate the old religion.
It wasn't until Theodosius I became emperor that Christianity, as articulated at Nicea and Constantinople became the official religion of the Roman Empire.
And while Nicene Christianity became the religion de juref of Rome in 391, Constantine's role was, while significant, not part of some radical restructuring of the theology of Christianity as most practiced it. Both sides in the theological debate saw themselves as heirs of what Christianity had practiced in the pre-Constantine era, most of the bishops who met at Nicea had lived and suffered through the reign of Diocletian and came to Nicea with the physical scars to prove it.
Many give entirely more credit to Constantine than he deserves.
By "politically motivated" I'm also referring to the simple idea of spreading a religion. Why is any religion created and spread? Religion and politics are always lumped together because they are in a way one in the same. They're both based on written law, personal beliefs, campaigning, and are the subject of extreme conflict and argument. The writers of the gospels were politically motivated to spread their religion, simple as that. I can't go into the minds of Matthew or Mark and know exactly what pushed them to write what they did, but it's fairly easy to believe that they did what they did to push their own agenda in spreading their ideology.
Of course the Gospels were written to spread the agenda of the Evangelists, that's in the very nature of gospel as a literary genera.
However, long before imperial politics became intertwined with Christianity (something I regard as the single worst thing to happen in the history of the Christian Church) Christianity had already spread quite abroad, not just within the Roman Empire as far as Britain, but had spread outside of Rome. Christian communities existed as far as Ethiopia, India and China, without any aid from Roman political power.
But the Old Testament contradicts with this sentiment. Do you acknowledge that Christ did not fulfill every prophecy God stated he would?
I believe Jesus fulfilled what needed to be fulfilled when He walked the dusty rodes of Judea. I agree with the early Christians who saw messianic fulfillment coming to fruition in Jesus.
-CryptoLutheran