Enter Silm (Islam) perfectly (Surah Al-Baqarah 2: 208-209)

Then if you slide back after the clear signs (Prophet Muhammad , and this Qur'an and Islam) have come to you, then know that Allah is All-Mighty, All-Wise. (Surah Al-Baqarah 2:209)


Entering Islam in its Entirety is obligated

Allah commands His servants who believe in Him and have faith in His Messenger (Sallallahu Alayhi wa Sallam) to implement all of Islam's legislation and law, to adhere to all of its commandments, as much as they can, and to refrain from all of its prohibitions. `Al-`Awfi said that Ibn `Abbas said, and also Mujahid, Tawus, Ad-Dahhak, `Ikrimah, Qatadah, As-Suddi and Ibn Zayd said that Allah's statement:

 Enter Silm  means Islam. Allah's statement:

perfectly means, in its entirety. This is the Tafsir of Ibn `Abbas, Mujahid, Abu Al-`Aliyah, `Ikrimah, Ar-Rabi` bin Anas, As-Suddi, Muqatil bin Hayyan, Qatadah and Ad-Dahhak. Mujahid said that the Ayah means, `Perform all the good works and the various pious deeds, this is especially addressed to those from among the People of the Scripture who embraced the faith.'
Ibn Abu Hatim reported that Ibn `Abbas (Radhi Allahu Anhuma) said that:

O you who believe! Enter Silm perfectly  refers to the believers among the People of the Scripture. This is because they believed in Allah, some of them still followed some parts of the Tawrah and the previous revelations. So Allah said:

Enter Islam perfectly.  Allah thus commanded them to embrace the legislation of the religion of Muhammad in its entirety and to avoid abandoning any part of it. They should no longer adhere to the Tawrah. Allah then said:

...and follow not the footsteps of Shaytan  meaning, perform the acts of worship and avoid what Satan commands you to do. This is because:

He (Shaytan) commands you only what is evil and Fahsha' (sinful), and that you should say about Allah what you know not. (Surah Al-Baqarah 2: 169)   and:

He only invites his Hizb (followers) that they may become the dwellers of the blazing Fire. (Surah Fatir 35:6)  Hence, Allah said:

 Verily, he is to you an open enemy.

Allah said:
Then if you slide back after the clear signs have come to you meaning, if you deviate from the Truth after clear proofs have been established against you,

...then know that Allah is All-Mighty   in His punishment, and no one can escape His vengeance or defeat Him.

All-Wise  in His decisions, actions and rulings. Hence Abu Al-`Aliyah, Qatadah and Ar-Rabi` bin Anas said, "He is Mighty in His vengeance, Wise in His decision.''

Source: Tafsir ibn Kathir (Abridged)