Entering Islam in its Entirety is obligated
Allah commands His servants who believe in Him and have faith in His Messenger 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:
[ادْخُلُواْ فِي السِّلْمِ]
(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 said that:
[يَـأَيُّهَا الَّذِينَ ءَامَنُواْ ادْخُلُواْ فِي السِّلْمِ كَآفَّةً]
(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:
[ادْخُلُواْ فِي السِّلْمِ كَآفَّةً]
(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:
[وَلاَ تَتَّبِعُواْ خُطُوَتِ الشَّيْطَـنِ]
(...and follow not the footsteps of Shaytan) meaning, perform the acts of worship and avoid what Satan commands you to do. This is because:
[إِنَّمَا يَأْمُرُكُم بِالسُّوءِ وَالْفَحْشَآءِ وَأَن تَقُولُواْ عَلَى اللَّهِ مَا لاَ تَعْلَمُونَ ]
(He (Shaytan) commands you only what is evil and Fahsha' (sinful), and that you should say about Allah what you know not.) (2:169) and:
[إِنَّمَا يَدْعُو حِزْبَهُ لِيَكُونُواْ مِنْ أَصْحَـبِ السَّعِيرِ]
(He only invites his Hizb (followers) that they may become the dwellers of the blazing Fire.) (35:6) Hence, Allah said:
[إِنَّهُ لَكُمْ عَدُوٌّ مُّبِينٌ]
(Verily, he is to you an open enemy.)
[فَإِن زَلَلْتُمْ مِّن بَعْدِ مَا جَآءَتْكُمُ الْبَيِّنَـتُ]
(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,
[فَاعْلَمُواْ أَنَّ اللَّهَ عَزِيزٌ]
(...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.''
[هَلْ يَنظُرُونَ إِلاَّ أَن يَأْتِيَهُمُ اللَّهُ فِي ظُلَلٍ مِّنَ الْغَمَامِ وَالْمَلَـئِكَةُ وَقُضِىَ الأَمْرُ وَإِلَى اللَّهِ تُرْجَعُ الأُمُورُ ]
(210. Do they then wait for anything other than that Allah should come to them over the shadows of the clouds and the angels (Then) the case would be already judged. And to Allah return all matters (for decision).)