There are five pillars of Islam: 1- The testimony that there is no god but Allah and that Muhammad is the Messenger of Allah; 2- Establishing regular prayer; 3- Paying the zakah; 4- Hajj (pilgrimage) and 5- Fasting Ramadan.

One of the greatest reasons for which Allah has created us – which is one of the greatest tests – is the command to affirm His Oneness (Tawhid) and to worship Him Alone with no partner or associate. For more, see the detailed answer.

The purpose behind the creation of man, the heaven and earth, is that Allah, may He be Glorified and Exalted, may be known and that His Oneness (Tawhid) may be affirmed and that He may be obeyed. Allah does not admit people to Paradise or Hell simply because He knows that they deserve that, rather He will admit them to Paradise or Hell on the basis of the deeds that they actually did in this world. If Allah created mankind and put them in His Hell, they would soon argue that He did not test them or give them the chance to strive. Allah wanted to refute this argument, so He created them in this world and gave them reason, and revealed His Books, and sent His Messengers; all of that is so that they will have no argument against Allah on the Day of Resurrection.

It is permissible for a scholar who has insight to read the Torah, Gospel and Zabur in order to refute Jewish and Christian opponents of Islam. Ordinary Muslims have no right to do that; rather if they have anything of the Torah or Gospel or Zabur, they must bury it in a clean place or burn it so that no one will be misguided by it

What is forbidden with regard to congratulating the Christians on the occasion of their festivals is expressing joy to them, being overly polite and showing approval of their actions, even if that is only expressed outwardly without feeling it inwardly.