Almight Allah Says in the holy Quran,"O ye who believe! Bow down and prostrat yourselves, and worship your Lord and do good, that haply ye may prosper. And strive for Allah the endeavour which is His right. He hath chosen you and hath not laid upon you in religion any hardship; the faith of your fathers Abraham (is yours). He hath named you Muslims of old time and in this( Scripture), that the messangedr may be witness against you, and that ye may be witness against mankind. So establish worship, pay the poor due and hold fast to Allah. He is your Protecting friend. A blessed Patron and a blessed Helper" (Al-Hajj:78).
Striving for Allah or Jihad has many catagories as: jihad by pen, oral jihad i.e. to enjoin to good and to forbid to evil, jihad by wealth and jihad by Nafs-i-Ammarah it means inordinate soul lies in human body.
The holy Prophet PBUH when he was returning from gazwa-e-bani Mustaliq, said to his companions (may God be pleaded with them) “return now to Jihadi Akbar from Jihadi Asghar". Because the holy war against the infidals takes place from time to time but this particular war is going on 24 hours agains Nafs-i-Ammarah and Satan, they try their best to mislead human being for every time. The holy Qur'an reveals,"I do not exculpate myself. Lo! the (human) soul enjoineth unto evil, save that wheron my Lord hath mercy. Lo! my Lord is forgiving, Merciful" (Joseph:53).
Statue of the human being was made by clay as the holy Qur'an reveals,"Verily We created man of potter's clay of black mud altered. And the Jinn did We created before time of assential fire" (Al-Hajar:27). The commentators say that the man has been made by four elements i.e. the fire, air, water and dust and their qualities are: hardness, weight, falling down, enmity and hate among each anothers etc. As we see in our routine life that the mankind makes hard struggle to gain his profitable stuff and to avoids from his harmful stuff and uses every lawful and unlawful methods to fulfill his lust and desires.
Similarly, due to falling down quality, he can fall from the high status of purity, piety or good guidance to the lower status of misleading Apostasy or infidelity etc. So" jihad Akbar" (against inordinate soul) is inevitable and most important lest the Muslim should fall in the dark of misleading.