"The religion of the Greeks was paganism."
Translation:Religia grecilor a fost păgânismul.
”Era” is also acceptable, since the English phrase gives no insight as to whether it ever stopped being (paganism). Since the imperfect is used when an action is in the past but does not have a defined end point (like past continuous in English), it most certainly can be translated as imperfect. The counterargument could be made that the Greeks dont practice paganism anymore and that this factor is implied in the past perfect definition that DL sticks to, but even in this case, from a morphosyntactic viewpoint there is no reason why the imperfect could not be used - and, the more I think of it, should not be used - in place of the given past perfect solution.