Part 3

"THEY ASK YOU ABOUT THE NEW MOONS. SAY: THEY ARE INDICATIVE OF TIME FOR THE PEOPLE, AND OF THE HAJJ. AND IT IS NOT RIGHTEOUSNESS THAT YOU COME INTO YOUR HOUSES FROM THEIR BACK SIDE BUT RIGHTEOUSNESS IS THAT ONE FEARS ALLAH. AND COME TO THE HOUSES THROUGH THEIR DOORS. AND FEAR ALLAH SO THAT YOU MAY BE SUCCESSFUL. (SURAH AL-BAQARAH:189)

Continued from before...

From the second half of this verse also comes the ruling that people should not start believing by their own opinion that certain acts are Wajib (necessary) or Ibadat (act of worship), if those acts have not been declared Wajib or Ibadah by Shariah. Similarly, if an act has been declared permissible by Shariah it is a sin to declare it a sin. This was exactly what had been done by the people of that time that they considered entering their houses from the front door, which was permissible, to be a sin, and considered entering after breaking down the rear wall which was unnecessary, to be necessary. 

The reason why Bid'at (declaring non-Ibadat Ibadat) have been forbidden is that it involves considering those acts necessary which have not been declared necessary by Shariah, and considering impermissible what has been declared permissible by Shariah.

From Ma'ariful Quran, by Mufti Muhammad Shafi RE