A very interesting set of quite different factors and historical influences came together, I recently learnt, in the fourth century to establish rich artistic traditions that allowed the Buddha, the Christ and the panoply of the Hindu gods to be portrayed in human forms for the first time and in a manner that was cultural diffuse and immediately recognisable. Though these movements took place around the same time, the religions were at different stages of development and acceptance at this point—what with the Brahmin’s gods already enjoying milennia of devotion, Siddhārtha Gautama having achieved enlightenment some eight hundred years prior and the latest incarnation of the Abrahamic faith in its fourth century. Despite these difference, they all started adopting pictorial representations around the same time.
A maturing network of international trade is of course a contributing factor, as being able to mediate on a shared image of how Jesus and company ought to look rather than relying on more abstract translated texts and interpreted teachings would spread these big religions and ensure their survival, but it is not the whole story. Before we got to the images of the serene Buddha and Jesus Christ in his characteristic poses, the story of these two was communicated through symbolism, teaching aides that represented the bodhi tree, the footprint of Buddha or the Cross, the sign of the fisher of men. And while it does seem natural and an effective step that the adherents of Buddhism would create figures of a limited and iconic variety for the benefit of foreigners being introduced to the philosophy, for Christianity it was a break with ancient traditions and taboos of not depicting God or His manifestations. The decision to show Jesus as a man may have happened in part because Constantine around this time declared that faith the official one of the empire, and Romans and Greeks, used to having statues of Dionysus, Hercules or Nike decorating their villas with triumphant flair, thought it was acceptable to have even more glorious statues of Jesus on display. As with Buddhism, the move was probably also good for the edification of foreign-speakers. Some three hundred years later, during the first few decades of the faith, Islam restored the proscription again representing the divine by human-hands by issuing currency for the Caliphate that only bore the word of God, instead of coins bearing the image of the head-of-state or other trappings.
Places of worship were becoming somewhat uniform in their delivery but the coin of the land was really the only mass-produced and reliable product of the Middle Ages in the West by any reckoning. Insisting on the rubic of a shared language was a powerful tool, and it is remarkable that this level of organisation developed in just as many decades as centuries it took for other religions—and without pictures. The Hindu gods and their different aspects were almost too innumerable to catalogue, but with the rise of the Gupta dynasty to power on the sub-continent at this same time, there was an ambitious and successful effort to standardise how each avatar looked and deported his- or herself. Because of this promotion and propaganda, one could communicate a certain devotion with a few accepted conceits. The personal nature of the gods and their care and custody would be instantly understood and copied. A sketch on a napkin being equally holy as any statue in a temple, and the image is understood to be the deity itself, to be treated as a honoured guest.
A very interesting set of quite different factors and historical influences came together, I recently learnt, in the fourth century to establish rich artistic traditions that allowed the Buddha, the Christ and the panoply of the Hindu gods to be portrayed in human forms for the first time and in a manner that was cultural diffuse and immediately recognisable. Though these movements took place around the same time, the religions were at different stages of development and acceptance at this point—what with the Brahmin’s gods already enjoying milennia of devotion, Siddhārtha Gautama having achieved enlightenment some eight hundred years prior and the latest incarnation of the Abrahamic faith in its fourth century. Despite these difference, they all started adopting pictorial representations around the same time. 
A maturing network of international trade is of course a contributing factor, as being able to mediate on a shared image of how Jesus and company ought to look rather than relying on more abstract translated texts and interpreted teachings would spread these big religions and ensure their survival, but it is not the whole story. Before we got to the images of the serene Buddha and Jesus Christ in his characteristic poses, the story of these two was communicated through symbolism, teaching aides that represented the bodhi tree, the footprint of Buddha or the Cross, the sign of the fisher of men. And while it does seem natural and an effective step that the adherents of Buddhism would create figures of a limited and iconic variety for the benefit of foreigners being introduced to the philosophy, for Christianity it was a break with ancient traditions and taboos of not depicting God or His manifestations. The decision to show Jesus as a man may have happened in part because Constantine around this time declared that faith the official one of the empire, and Romans and Greeks, used to having statues of Dionysus, Hercules or Nike decorating their villas with triumphant flair, thought it was acceptable to have even more glorious statues of Jesus on display. As with Buddhism, the move was probably also good for the edification of foreign-speakers. Some three hundred years later, during the first few decades of the faith, Islam restored the proscription again representing the divine by human-hands by issuing currency for the Caliphate that only bore the word of God, instead of coins bearing the image of the head-of-state or other trappings. 