Here from the Tafsir of Ibn Kathir :
6.159 :
Mujahid, Qatadah, Ad-Dahhak and As-Suddi said that this Ayah was revealed about the Jews and Christians. Al-`Awfi said that Ibn `Abbas commented,
Before Muhammad was sent, the Jews and Christians disputed and divided into sects. When Muhammad was sent, Allah revealed to him,
It is apparent that this Ayah refers to all those who defy the religion of Allah, or revert from it.
Allah sent His Messenger with guidance and the religion of truth so that He makes it victorious and dominant above all religions.
His Law is one and does not contain any contradiction or incongruity.
Therefore, those who dispute in the religion, religious sects, just like those who follow the various sects, desires and misguidance - then Allah has purified His Messenger from their ways. In a similar Ayah,
Allah said, This, indeed, is the straight path which the Messengers have brought and which commands
worshipping Allah alone without partners and adhering to the Law of the last Messenger whom Allah sent. All other paths are types of misguidance, ignorance, sheer opinion and desires; and as such, the Messengers are free from them.
http://www.qtafsir.com/index.php?option=com_content&task=view&id=941&Itemid=61
30.31 :
"and be not of the idolators." Be of those who single out Allah, devoting their worship sincerely for Him alone, not anyone or anything other than Him.
Ibn Jarir recorded that Yazid bin Abi Maryam said: "`Umar, may Allah be pleased with him, passed by Mu`adh bin Jabal and asked, `What is the foundation of this Ummah' Mu`adh said,
`Three things, and they are the things that will bring salvation:
-Al-Ikhlas (doing a deed for Allah alone), which is the Fitrah with which Allah created mankind;
-Salah, which is the thing that tells a believer apart from a disbeliever;
-and obedience, which is protection.'
`Umar said: `You have spoken the truth.
http://www.qtafsir.com/index.php?option=com_content&task=view&id=1769&Itemid=86
30.32 :
"Of those who Farraqu Dinahum (split up their religion), and became sects, each sect rejoicing in that which is with it." means, do not be of the idolators who split up their religion, i.e.,
changed it by believing in parts of it and rejecting other parts.
Some scholars read this as Faraqu Dinahum, meaning
"neglected their religion and left it behind them.'' These are like the Jews, Christians, Zoroastrians, idol worshippers and all the followers of false religions, besides the followers of Islam, as Allah says 6.159
The followers of the religions before us had differences of opinions and split into
false sects, each group claiming to be following the truth.
This Ummah too has split into sects, all of which are misguided apart from one, which is Ahlus-Sunnah Wal-Jama`ah,
those who adhere to the Book of Allah and the Sunnah of the Messenger of Allah and what was followed by the first generations, the Companions, their followers, and the Imams of the Muslims of earlier and later times.
In his Mustadrak, Al-Hakim recorded that
the Messenger of Allah was asked which of the sects was the saved sect and he said:
"What I and my Companions are upon."
http://www.qtafsir.com/index.php?option=com_content&task=view&id=1768&Itemid=86
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Allah said :
33.21 There has certainly been for you in the Messenger of Allah an excellent pattern for anyone whose hope is in Allah and the Last Day and [who] remembers Allah often.
What is sunna :
https://en.wikipedia.org/wiki/Sunnah
) have no concern in them in the least. Their affair is only with Allah, Who then will tell them what they used to do. 
