Which team besides windies, zimbabwe and bangladesh can india defeat in test series?

I doubt anyone else besides these 3 teams can India win in test series. India appears to be ranked 7th or 8th in test rankings from number 1 ranking last year. What is the cause of this severe decline? It has to be T20, right? this is at home. overseas they may even lose to windies now in tests the way the team appears.