Mankind was one nation, believing in Tawheed (Monotheism) , then they differed. Some of them believed and some disbelieved. So Allah sent the Prophets with glad tidings and warnings, so whoever believes will enter Paradise and whoever disbelieves will enter Hell. The conflict is still going on between belief and disbelief, truth and falsehood, and will continue until Allah inherits the earth and everyone on it.
Islam is the religion for all of mankind, and Allah has commanded us to convey it to all people. This can only be achieved if we are strong, and strength is based on faith and unity. Hence Allah has commanded all the believers to adhere to His religion and to be united, and not to be divided. Allah says (interpretation of the meaning):
“And hold fast, all of you together, to the Rope of Allah (i.e. this Qur’aan), and be not divided among yourselves" [Aal ‘Imraan 3:103]
Division, differences and disputes are the cause of the ummah’s defeat, as Allah says (interpretation of the meaning):
“And obey Allah and His Messenger, and do not dispute (with one another) lest you lose courage and your strength departs, and be patient. Surely, Allah is with those who are As-Saabiroon (the patient)” [al-Anfaal 8:46]
Unity and coming together are among the basic principles of Islam, and there are many aspects of unity in Islam, such as One Lord, one Book, one Prophet, one religion, one qiblah, one ummah.
In order to achieve the unity of the ummah, Islam urges us to adhere to the jamaa’ah (the group which follows the Qur’aan and Sunnah). The Messenger (peace and blessings of Allah be upon him) explained that the hand of Allah is with the jamaa’ah, and that whoever deviates from that will be in Hell. Allah has enjoined coming together for all acts of worship in order to achieve this unity. Allah addresses the ummah as one group in all rulings to indicate that they are one ummah, like one body. There is no difference between them; the commands and prohibitions are addressed to all.
In the context of worship, Allah says (interpretation of the meaning):
“Worship Allah and join none with Him (in worship) ” [al-Nisaa’ 4:36]
And He enjoins them all to establish regular prayer:
“Guard strictly (five obligatory) As-Salawaat (the prayers) especially the middle Salaah (i.e. the best prayer - ‘Asr). And stand before Allah with obedience” [al-Baqarah 2:238]
Concerning zakaah, Allah commands them all, as He says (interpretation of the meaning):
“And perform As-Salaah (Iqaamat-as-Salaah), and give Zakaah” [al-Baqarah 2:43]
Concerning fasting, He says (interpretation of the meaning):
“O you who believe! Observing As-Sawm (the fasting) is prescribed for you as it was prescribed for those before you, that you may become Al-Muttaqoon (the pious)” [al-Baqarah 2:183]
Concerning Hajj, Allah says (interpretation of the meaning):
“And Hajj (pilgrimage to Makkah) to the House (Ka‘bah) is a duty that mankind owes to Allah, those who can afford the expenses (for one’s conveyance, provision and residence)” [Aal ‘Imraan 3:97]
Concerning jihaad, Allah says (interpretation of the meaning):
“And strive hard in Allah’s Cause as you ought to strive (with sincerity and with all your efforts that His Name should be superior)” [al-Hajj 22:78]
Islam regards all people as equal before the laws of Allah. Black and white, Arab and non-Arab, male and female, rich and poor – Islam brings them all together and addresses its commands and prohibitions to them all. So whoever obeys Allah and His Messenger will enter Paradise and whoever disobeys Allah and His Messenger will enter Hell, as He says (interpretation of the meaning):
“Whosoever does righteous good deed, it is for (the benefit of) his ownself; and whosoever does evil, it is against his ownself. And your Lord is not at all unjust to (His) slaves” [Fussilat 41:46]
From Usool al-Deen al-Islami by Shaykh Muhammad ibn Ibraaheem al-Tuwayjri