Inter-religion Similarities

Today when we look at prevailing rituals in different religions, we realise the similarities between them, irrespective of the monotheistic or polytheistic religious philosophy.
I believe the reason behind it is that every religion which exist now or the religions which are dead now, they all got the basic learning of their belief from one common source, a source which is known by different names in different religions.
Monotheist believe in one God, and that one God of monotheist is also at the core of polytheistic religion, only difference being that human since birth are prone to believe on things which they can physically see, to make them sound more confident of their belief they started creating Gods and Goddesses for their every need. This lack of faith gave rise to mythical stories related to Gods, which ultimately paved way for mythical religion of past and present.
Going by the concept of monotheistic belief i can say that since beginning of time there was only God, and that God conveyed the same message to every human through His prophets, messengers, saints etc. Human due to their lack of understanding and faith, started creating their own Gods and forgot the teaching of that one God which were conveyed to them by their ancestors.  I feel this is the primary reason that today we see the similarities in various rituals performed by the followers of different religions.