Don't think he'd be an atheist. He himself had been converted to a Puritan protestantism when still a young man and got more radical as he got older.
He'd have his own political party.
He'd be puzzled by modern society, that's certain ! He'd have thought us immoral and sinful.He'd be gratified that the monarchy had no power and that Parliament was powerful. For some reason he is seen as a great parliamentarian but in reality his parliamentarianism was a necessary part of his rejection of King Charles' belief in the divine right of kings to rule without question.Once in power Cromwell did not seem to be particularly parliament minded . If Charles had not been such an idiot (easy to say with the benefit of hindsight !) there was a fair chance that Charles could have lived a low key life, without civil wars and quietly accepted a back seat, because Cromwell was pragmatic.In fact, had Charles been more realistic ,Cromwell himself might never have become what he did, but just an important member of parliament.