they are not worshipping God. And there is only one God (who is both Father, Son & Holy Spirit)
simple example: Christianity says Christ is God; Islam says Christ was merely a prophet. Christianity says God has a son named Jesus; Islam says there is no son of God.
In this instance, only one can be true, Christianity or Islam. They are mutually exclusive. Of course some will say both are wrong (eg there is no God) but that is not the point here.
Christianity shares much common ground with other major religions (e.g. be good, don't murder etc) but it is the differences that matter, since they cannot all be true. Simple logic.
Worshipping nothing? Well, in order not to be divisive, let me just say the important point is that they are not worshipping the God of the Bible. They are worshipping someone / something else.
That's what is so bogus about the new agey stuff so common today, the idea that all roads lead to Rome. Utter garbage. THAT is choosing not to think, being afraid of conflict, pick n mix belief. From a sceptics POV, if there is a God, that means there is an absolute truth, in which case he must be consistent.