What was the intended purpose of the frescoes from the house in Dura-Europos?

Dura-Europos was an ancient city located in the vicinity of village of Salhiyé, Syria, along the Euphrates river, close to the Iraq border.

Christian places needed to be discreet or secret around 240 A. D.; thus earlier congregations worshiped in private houses of wealthy members; some of those houses were converted into churches, and the house in Dura-Europos was one of those conversions.

So the purpose of the frescoes was to celebrate Christian worship secretly, and to illustrate the old testament writings, mostly for the illiterate people who couldn't read the sacred books, as a medium of what they called "hope of new spiritual birth".