First off, to anyone who might get butthurt over this since this is the deleting of a world religion (perhaps even more religions gone), remember that this is alternate history, not real history. But now let's get to it.
What if Christianity had never existed? Surely it would affect the world, since another world religion, Islam, had Christian roots because of the monotheism and such. With that, 3,500,000,000 people maybe without religion would definitely affect something. It is one-half of the current population in the world. We're going to talk about other aspects of how it would affect the world in this and other pages.
How Christianity Wouldn't Exist
In our current timeline, Judaism was an important religion in the Fertile Crescent. It would be the same here, except with the Roman invasion of Judea in 6 AD, Romans killed all the Jews in their province. They are then seen as a cult, with at least 1,000 still remaining. And let's say the Romans further invaded the Middle East before this even happened, leaving modern-day Israel alone, not just in Syria, but up to modern-day Afghanistan and Pakistan. So Roman mythology would be the ruling religion in the Middle East. Which brings us to Joseph and Mary, who were both Jews. Because they lived in modern-day Israel they would be killed off, and possibly their kid Jesus, too. If Jesus survived, he would have been killed off much earlier before he could bring up 50,000 followers. That would be because maybe Jesus was part of the small cult of Judaism. Then again, before the 50,000 people converted, they were Jews, so they may not exist either. So in the end, the 1,000 Jews still left either die from disease brought by Romans or they are rounded up and killed. (Again, not anti-Semitic.)
What About The Romans?
Because of expansionism of the Middle East and the eradication of Jews, Rome has become a mightier power. Now let's say the 53 BC Battle of Carrhae between Rome and the Parthians ended in a victory instead of a defeat. So they would have taken some Parthian land. The main religion of the Parthian Empire was Zoroastrianism so they would have been safe from Roman prejudice for now. But let's say after lengthy wars with the Parthian empire, Rome takes over. Roman mythology being poly-theistic, they would think that Zoroastrianism is wrong so, like the Jews before them, the Zoroastrians were eradicated.
In the current timeline, Christianity was implemented as the religion of the Roman Empire. It was also cursed as one reason among others that Rome fell. What if, thanks to no Christianity, Rome did not fall?
Of course, there are other reasons Rome fell, such as decay of the military, the embracing of Barbarians, inflation, the hoarding of bullion, and the excess wealth of the Emperor. By the time of Claudius II Gothicus (268-270 AD), the worth of a supposedly 100% pure silver denarius was only worth 1/50th of 1% of the silver. This is because Nero and other emperors debased their currency, which means that instead of a coin having its own intrinsic value, it was going to be worth how much gold or silver it was in. If they didn't do this, inflation would be no problem. So in the alternate timeline, they had the right mind to not do such.
There is also the problem with Barbarians. In the current timeline, they let them into their ranks, which caused both decay of military and revolts against the Emperor. Let's say they didn't and they take over northern Europe, including Denmark, Northern Germany, Scandinavia, the Baltics, the Balkans, Poland, Ukraine, and parts of Russia after 117 AD. So Rome would become the major European power throughout the centuries. There is no Catholic Church to stop them, and all problems were settled when the Emperor decided to share some of his wealth with the poor of the Empire. He's still very very powerful and still an absolute leader.
To anyone who's going to read this, there will be more in the weeks to come.