Sheikh Mohamed Najmodin al-Kordi said: "I have divided the book into two parts: Sahih Al 'Aqeeda (the correct faith) and Sahih Ash-shari'a (the correct Islamic rules): Part one is on Al 'Aqeeda', which includes the beliefs of the Muslims which are the basis of the correct worship because, if one's doctrine is not correct, of course the worship itself will be incorrect. So, it is the doctrine itself which makes the difference between people on the Doomsday.
So, the disbelievers used to mock at the believers and now they don't find those believers with them in Hell. They say to themselves, "Maybe they are with us in Hell but we can't see them". Then, they know that they are in high ranks of Paradise,
The correct doctrine is that one which gives us correct understanding of Allah's Attributes, and the criterion which tells us if a certain doctrine is correct or not