The period of the system of Mahdi is a long one. The reason this period will be as long as 40 years is so that it possesses a great spiritual merit. Otherwise, God could establish the reign of Islam in one year if He wished so. However, the believers would earn one year’s worth of spiritual merit in that case. God extended the period of the system of Mahdi to 40 years so that it has great spiritual merit. - Harun Yahya