If Islam is the truest religion of God then why all the people in the world are not Muslims?
It is commonly held that Almighty allows religious guidance to some people and prevents others from getting to it on the basis of His own sole discretion. This understanding makes people stay away from making personal efforts to acquire and strengthen faith, because they contend that they have no role to play in either accepting faith or in rejecting it.
The Qur'an has mentioned the outlines of the law of the guidance very concisely in one of the earliest revealed surahs thus: "Yet he who spends on others and is righteous and affirms goodness, We shall ease the way of fortune for him. But he who does not spend and is unconcerned and rejects goodness, for him We shall ease the way of adversity." (2: 5-10)
God has created man with inborn guidance of good and evil. Everyone comes into the world with this awareness. He feels aversion to the evil and cherishes the good. He is fully granted freedom to choose between right and wrong. It is only he who decides whether to remain truthful or become deceitful in a particular situation. Not only this but he is also bestowed with the conscience, which reprimands and reproaches him on doing anything wrong. This sense grows strong if continuously paid heed to. However if one unashamedly ignores the call of the conscience it becomes weaker and weaker until it stops rebuking any more. This is the worst condition when there is no hope for anyone to accept the truth. The matter of religion is just the same as other matters. You are an addressee of a prophet of God who unveils the truth before you in its ultimate form and it becomes so manifest before as it could be then you will be among first who will embrace it provided you have been acting upon the inner guidance and heeding to the conscience. But if you had ignored both the inborn knowledge of good and evil and suppressed the rebukes of your conscience then it will be difficult for you to embrace the truth.
But in absence of a prophet, if you are invited affectively and convincingly to the teaching of Islam then your inner guidance will lead you to accept it. But now much depends upon the style and argumentation on the part of the person calling to the faith. If successfully makes you clear about his call and you are convinced of the truth of what is being presented then you will be held responsible if you act contrarily. You are required to follow whatever you are convinced of being truth and steadfast in it in all circumstances.
The reasons why all the people of the world are not Muslims are that sometimes people have corrupted both their inner guidance and conscience and are no more able to accept any truth no matter how manifest and pronounced it is. Sometimes the call for a particular faith is not presented affectively and it fails to convince others. Sometimes it is presented in proper a fashion.