Why people are harping so much about borrowed religion?

I’ve not heard these people. Who are you talking about? What is borrowed religion?

How much is it to borrow?

Just a wild "stab in the dark here" . . . Do you mean "Borrowed religion" as in a religion being just a plagiarized copy of another older religion?

I can't think of another use for that phrase . . .

It is interesting how religions develop. The Judeo-Christian religions almost  certainly grew from the sun god (Yes, there are more sorts of sun-worshippers than those on the beach). Likewise Islam is probably an extension of moon worship. Both religions have been shaped and reshaped over time, along the way, and are still evolving, as new (and human) interpretations of "gods word" are put forward. As a frequent visitor to India, I was always fascinated by the way the same "faith" was interpreted differently in different places, sometimes changing with a 10 mile bus ride. Hinduism has many forms.

If you search "borrowed religions," here a few results you get under "searches related":

---Christianity stolen from other religions

---list of things Christianity stole from paganism

---ancient Egyptian religion influence on Christianity

---Christianity borrowed from other religions

The general thrust seems to be that Christianity is no more than a compilation of mankind's attempt to understand things that can't be explained easily along with some rules to structure (in some sense) "moral" behavior.

Of course, anyone who is a good parent knows that you have a final goal in raising any child---to help him to become a fully functioning adult. 

And that requires a plan implemented over the life of the child as he grows.

Which in turn involves rules and procedures to the child appropriate to his ability to understand and accept various "rules" of living.

If I were God (and there are philosophical principles that argue conclusively that I am not), if my creation rebelled against me by improperly exercising its free will and I intended to reestablish a new  relationship with them through Christ, I would have given them a number of ideas which would prepare creation by sending them numerous hints of things that I would eventually bring together in the Church I would establish.

In other words, while I established a chosen people, I was already making it easy for the gentiles to accept me by allowing other experience of the "good" with them.