It is because God Almighty says in this Ayah...

It is because God Almighty says in this Ayah, "Therefore stand firm (in the straight Path) as you are commanded." Standing firm and putting up a resistance is a difficult thing to do. This resistance is the same as the Bridge of Sirat which we have to cross on Judgment Day. In this world, our actions are like the Bridge of Sirat. We are walking on the Bridge of Sirat now. We should be very careful. If each person remains so careful in everything he does, he will soon grow old.

In my opinion, the most important part of this Ayah is this part: "And those who with you turn (unto Allah)". The Holy Prophet (S) is not just responsible for his own resistance. He should make the massive number of believers put up a resistance.

He is responsible for people who, on the one hand, are faced with the daily problems of life such as the problems that the enemies, the plotters, the ill-wishers and the dominant powers create for them and who, on the other hand, are subject to their carnal desires for worldly profits which attract people strongly. He is responsible for people who easily deviate from the right path.

The desire for gold and silver, the desire for money, the desire for sexual pleasure, the desire for power and other desires, tie a rope around their necks and pull them toward themselves. Most probably, helping people put up a resistance against these temptations, guiding believers towards the right path and keeping them on this path were the things which made the Holy Prophet (S) old.

Believers may deviate from the right path under the influence of these two strong forces: the enemies and their inner passions and desires. Do you know what the Holy Prophet (S) did both when he was living in Mecca for thirteen years and when he was forming a government in Medina to help believers tread many difficult paths and reach the peak of humanity? Nobody could do such a thing. He changed people who had no understanding of anything and who did not have any human virtues.

The Holy Prophet (S) changed them into such noble people whose great and enlightened hearts made God's angels feel inferior. This is a perfect example of resistance. Today we need resistance. We can also be tempted by different desires.