“At the time, prosperity ministry and the promise of riches from heaven was popular.”
Ministry is one noun, promise is another noun, so we have a pretty clear plural going here.
A plural requires were rather than was.
Of course, if a noun combination is regarded as all one thing — mac and cheese, for example — then you can treat it as singular and use was. You would never say the mac and cheese were yummy. The mac and cheese was yummy.
But if you regard “prosperity ministry and the promise of riches from heaven” as all one thing, well … you’re wrong. On multiple levels.
English Is A Complicated Language 