I found this sweet sakhi on Sikh sangat.com
its really interesting =)

Once Bahadar Shah asked Guru Gobind Singh Jee:
“In Islam it is believed that if any person, despite how deviant, evil or sinful becomes a Muslim and instills withing faith on the Prophet Muhammad, reads the Kalma (verse from the Qu’ran) that in the Court of Allah that person’s sins and misdeeds will be washed away. Guru Sahib what are your views on this?”

Before answering, Guru Gobind Singh Sahib pulled out a coin from his pocket. He then gave it to his Sikh and signaled him to buy something from the Bazar (Market) with it.

A few minutes later, the Sikh returned back from the market. The Sikh came to Guru Sahib and said:
“Maharaj, this coin was not accepted at the Market.”

Satguru Jee picked up the coin and looking at said, “Why is this coin not accepted at the Bazar? It is made by the Emperor’s mint, and Bahadar Shah’s stamp is on it. Its outward appearance is also okay. So why wasn’t it accepted at the Bazar? Pass the coin to Bahadar Shah for him to have a look at it ask him to tell us what is wrong or lacking in this coin.”

It is said that while holding and examining the coin, Bahadar Shah said to Guru Jee, “Maharaj it is true that this coin was minted in my taksal (mint), that it is has been stamped and that the print is perfectly correct. Everything is okay. However despite this, the coin will be accepted in the Market because however the amount of pure gold there should be coating the coin, is insufficient in this one . Basically its inside is false, wrong.”

Guru Gobind Singh Jee then said to Bahadar Shah, “Badshah, you have now received the answer to your question.”

He replied:
“Maharaj, I don’t understand. Please explain to me in deeper detail.”

Satguru Jee said:
“Despite, whichever taksal (mint) the coin is minted and stamped by, If the coin is “kohtta”, false or inaccurate inside, it will not be accepted in the market. Similiarly if a person is “kohtta”, false, inside then he will not be accepted in the Court of Waheguru, despite whether he has the stamp of being a Muslim, Hindu or Sikh. Whether stamped as Christian or labelled as another religion, if we are not pure inside then we are not worthy of being accepted in God’s court.”