The important part of a building is the foundation, if the foundation is strong, the building will be strong too. In Islam it is exactly the same: if your pillars (Shahadah, Salah, Zakah, Sawm and Haj) are strong then your other fundamentals will also be sound.
The two angels who are charged with questioning the dead come to him and ask him what he used to believe in his earthly life, who was his Lord, what was his religion and who was his Prophet. If he gives a good answer, that is good, but if he does not answer them they inflict a severe and painful beating on him. (Tirmidhi) These are the three basic principles which every Muslim should know: knowledge about his creator (Allah), knowledge about his religion (Islam), knowledge about his Prophet Muhammad (peace be upon him). These are the answers to the questions which will be asked by everyone in the grave.
Allah, the Merciful has made the final test easy for us by telling us the questions. We just have to work hard toward it so that we can remember and answer them because our final test can be any time. We have to seek knowledge about these questions without associating any partners with Allah. Prophet Muhammad (peace be upon him) said, “Allah has promised that anyone who says this three times every morning or evening will be happy on the Day of Resurrection.” “I am pleased with Allah as my Lord,with Islam as my religion and with Muhammad (peace be upon him) as my Prophet.” For more read here.