And be not as those who divided and differed among themselves after the clear proofs had come to them. It is they for whom there is an awful torment.(3:105)

And one of the reasons why Muslims are divided into so many different sects is because, we started listening to individual's blindly, and forgot about following the Qur'an and the Sunnah. 

If Muslims went to see unity amongst themselves, and want to make sure they are the on the right path, then it is important only to follow the two sources, the Qur'an and the Sunnah of the Prophet (SAW), and not to follow people blindly, because any imam, mufti, a'alim, are all human beings, and are all bound to make mistakes, except for Allah and his Messenger (SAW).

The Qur'an raises and answers the question. And if one cannot find it in the Qur'an then look at the examples of the Prophet (SAW), they are thousands of guidelines in the Qur'an and in the Hadeeth, and statements from the companions of the Prophet (SAW). And if one does not, then is in the hands of Satan, who can misguide him, and misguidance leads towards hell. We spend 15-20 years in education in this world, which is left behind when we depart from this world, and all of this is only human research full of errors, and that is the reason why courses and computers always need upgrade. However what Allah and his Prophet (SAW) said is the final and most practical until the day of Judgment.

Every Muslim agrees that the most best Muslims are the companions of the Prophet (SAW). Now one may ask why? 

The reason behind this is the companions only followed what Allah and his Messenger (SAW) said, and today the Muslims have no idea what the Quran and the Sunnah are saying, and we only follow the teachings of individuals blindly.

…(And) if you differ in anything amongst yourselves, refer it to Allah and His Messenger, if you believe in Allah and the Last Day. That is better and more suitable for final determination. -Surah An-Nisaa:59

Say (O Muhammad): If you (really) love Allah, then follow me, Allah will love you and forgive you your sins -Surah Ali-Imran: 31

Whoever obeys the Messenger, has obeyed Allah-Surah An-Nisaa:80

The following are verses from the Holy Qur'an which give us full details regarding this matter,

Verily, those who divide their religion and break up into sects (all kinds of religious sects)^[], you (O Muhammad SAW) have no concern in them in the least. Their affair is only with Allâh, Who then will tell them what they used to do. (6:159)

So set you (O Muhammad SAW) your face towards the religion of pure Islâmic Monotheism Hanifa (worship none but Allâh Alone) Allâh's Fitrah (i.e. Allâh's Islâmic Monotheism), with which He has created mankind. No change let there be in Khalq­illâh (i.e. the Religion of Allâh Islâmic Monotheism), that is the straight religion, but most of men know not 

Of those who split up their religion (i.e. who left the true Islâmic Monotheism), and became sects, [i.e. they invented new things in the religion (Bid'ah), and followed their vain desires], each sect rejoicing in that which is with it(30:30,32)

The last sermon delivered by the Prophet (SAW) was like a conclusion of his lifetime. In the conclusion of the last sermon of the Prophet (SAW), he said:

O People, NO PROPHET OR APOSTLE WILL COME AFTER ME AND NO NEW FAITH WILL BE BORN. Reason well, therefore, O People, and understand words which I convey to you. I leave behind me two things, the QURAN and my example, the SUNNAH and if you follow these you will never go astray.

And to guarantee one is on the straight path then we should only hold on to these two sources, inshallah.