What were the chief historical trends that led to the emergence of these universal religions of salvation?
Several universal religions and philosophies of salvation emerged almost simultaneously in the Hellenistic world. This can be seen as part of a broader phenomenon of the growth of cosmopolitanism.
In the classical Greek and Republican Roman periods, religious and civic identity were essentially regional, tied to the local polis. One worshipped the local gods and in return they gave to polis various forms of favour. The increases in trade and travel, the unification of Greece under the Macedonians, and the expansion of the Roman empire all led, paradoxically, to both increased individualism and universalism. Although the individual polis was still a locus of civic pride (e.g. Libanius and Antioch, Plutarch and Delphi), cultural identity became more universal (Hellene vs. barbarian, paideia vs. illiteracy) and religion became more part of universal traditions than individual local deities. The mystery religions, Platonism, Pythagoreanism, Mithraism, Christianity, Manicheism, gnosticism, worship of Sarapis, etc. were paths simultaneously to individual salvation and universal rather than rooted in local deities.
They provided a mode of belief for the individual to cope with a rapidly changing multilingual, multicultural world.
People started realizing that the gods are not with them. That only one god the unseen god is here. Well the belief in salvation goes all the way to ancient egypt and even the first civilization the sumerians. Utnapishtim in sumerian texts supposedly died for to give us paradise. As he grew into what genesis refers as the garden of eden they call Akwan (good place) Well the egyptians are alot different. They had a belief that to go to paradise you must be pure of heart and relinquish your soul to the underworld. The egyptians believed that you can only be saved by your actions in life. Usually only the blood line of the kings went to paradise, yet the good where allowed if you where wise and pure of heart and even this was a rare occurance. The romans believed paradise was for those whom released their spirit to the gods. The Romans and greeks differ in one way. The greeks everyone went to heaven the romans on the other hand where really different they where more convert or die. The romans believed that patriotism for the coutries religion let you go to heaven. They thought the Christians and Jews where not patriotic so they persecuted them and became even jeoulos of them for there great influence. When somebody wants salvation they must find it themselves. Jesus taught to love your enemies and to end capitalism (worship of materialism) to favor only god. So the materialistic people then felt maybe this isn't good enough. They then where saved and ended there materialistic lifestyles. I believe materialism is what brings down empires. once those materials disapear so does it remaining citizens. Salvation is not done by the sword its done by the belief structures that make peace with our eternal souls.