Preserving Islam is by learning it, teaching it, spreading it and defending it
Question

Some people say that as Allah has guaranteed preserving this religion Himself, then all the work of the preachers for spreading and teaching Islam is useless and unneeded. How to answer those people?.
Praise be to Allah.The response to these people is simple: their attitude is the attitude of one who denies the principle of cause and effect, and there is no doubt that denying this principle is misguidance from Islam and foolish thinking. 
Allaah has guaranteed to preserve this religion, but that is through means and causes, by means of what the daa’iyahs do by spreading this religion and explaining it to people and calling people to it. This view is like that of those who say: Do not get married, and if it is decreed that you are to have a child, it will come to you; do not strive to seek provision for if provision is decreed for you it will come to you! 
We know that when Allaah said (interpretation of the meaning): “Verily, We, it is We Who have sent down the Dhikr (i.e. the Qur’aan) and surely, We will guard it (from corruption)” [al-Hijr 15:9], that was because He knew, as He is most wise, that things only happen due to causes. So Allaah decreed that this religion would be preserved through the means and causes that achieve this preservation. hence we find that the scholars of the early generations of Islam, when Allaah protected His religion from ideological and practical innovations, began to speak and write and explain to people. So it is essential that we do that which Allaah has enjoined upon us of defending Islam, protecting it and spreading it among people. Thus the required protection will be achieved. End quote.