Sorry, but viewed solely as a man, it wasn’t Jesus. 

As a man, Jesus was a mere itinerant Jewish zealot who left behind some inspiring but occasionally garbled and contradictory parables. 

Christianity is now the largest religion in the world, but the religion itself was chiefly created by those who came after Jesus, most prominently Paul and then Constantine. 

Paul, who converted 20 years after the death of Jesus (he never knew him), (i) opened up the religion to gentiles (to the horror of Jesus’s brother, James, who had inherited leadership of a still primarily Jewish cult, which fortuitously was fatally weakened in 70 AD with the Roman sack of Jerusalem), and (ii) imbued Christianity with Greek philosophical concepts. 

300 years after the death of Christ, when the adherents of the still very slow-growing Christianity were less than 5% of the population of Rome, Constantine became Emperor with the help of Christian troops. After trying to combine Christianity (the religion of his mother) with his own, Mithraism (a much more popular, eastern mystery religion involving a sun god dying and being born each day–his birthday being 25 December), Constantine ended up requiring anyone who wanted to hold office in the Empire to be a Christian, and at the same time at the Council of Nicaea, imposed order on the then embarrassingly heterodox Christian religion, including deciding which of the numerous existing Christian books and of each of those which of the many versions would be included in the New Testament.

Mohammed, on the other hand, not only wrote the book by himself–or more precisely dictated disparate inspirations which were, after his death, put together as the Koran–but also set the extremely fast-growing Islamic empire on its way.

Why did Mohammed’s Islamic empire expand so rapidly? There are several reasons:

• Mohammed utilized the Arab raiding tradition. He allowed his lieutenants to attack nonMuslims, as long as his lieutenants gave 20% of the proceeds to Islam. 
• Newly converted Islamic people received protection. As his power grew, Mohammed made clear to his lieutenants that they could not attack anyone who converted to Islam, thereby encouraging nonMuslins to convert and at the same time forcing Mohammed’s lieutenants to search wider and wider for non-Muslims to attack.
• Islam’s two main Imperial foes standing in Islam’s way, Christian Constantinople and Zoroastrian Persia, had been weakened by decades of fighting each other, by civil war in Persia, and by plague. The bubonic plague which had started in the previous century and continued sporadically into the next killed a significant portion of their populations. However, the infected rodents which thrived in the metropolitan areas of the two empires did not live in Mohammed’s nonmetropolitan desert. Result: the Persian Empire Zorastrianism was replaced by Islam, and Constantinople, while keeping its Christianity, lost a lot of territory.
• Early Islamic leaders showed tremendous military skill. Mohammed’s troops were led by the brilliant military leader, Kahlid Ibn al-Walid. He would likely have been more successful had not Umar, one of the prophet’s successors, removed the general for being too big for his britches. 
• Islam encouraged trade, cooperation, and economic viability. The religion Mohammed developed was, for its time, egalitarian, modern, and benign. It even had a place for Christians and Jews as people of the book. Moreover, Mohammed and his first wife were prominent traders, so the Koran is filled with rules on fair trading and dispute resolution. This meant that no matter where Muslims went in the empire, their religion required them to trade fairly under those God-declared laws. This caused a trade boom and gave the Muslim traders a reputation for honesty, which attracted foreign princes to them. Unlike the Christians, wealth creation was not seen as an impediment to entering heaven.

In short: Mohammed was in the right place at the right time with the right strategy and the right lieutenants. 

Jesus, on the other hand, was in the wrong place at the wrong time with the wrong strategy and the wrong lieutenants. It was only those who joined the church years after his death who eventually lucked out, expanding Christianity beyond the Jews, and then three centuries later, rocketing the religion up to become the sole religion endorsed by the Emperor. 

God works his wonders in mysterious ways, doesn’t he?

I often wonder about all those people who died without hope of salvation while Christianity waited around for its ultimate lucky break.