A Christian is asking why God created mankind

Praise be to Allaah. 

Allaah created the jinn and mankind to worship Him alone, to obey His commands and heed His prohibitions. Allaah says [in the Qur’aan] (interpretation of the meaning):

“And I (Allaah) created not the jinn and mankind except that they should worship Me (Alone)” [al-Dhaariyaat 51:56]

i.e., to believe in His Oneness and to devote their worship to Him alone, to the exclusion of all others. One of the conditions of worship being correct is belief in His Messenger [Muhammad] (peace and blessings of Allaah be upon him) and following his example, for he is the example to be followed, as Allaah says (interpretation of the meaning):

“Indeed in the Messenger of Allaah (Muhammad) you have a good example to follow [al-Ahzaab 33:21]