O you who have believed, enter into Islam perfectly (extract from Tafsir As-Sa'dee rahimahul-llah)

 Imam Abdur-Rahman bin Naasir As-Sa'dee rahimahul-llah (died 1376) said in his tafsir of verse 208 in Suratul-Baqarah: 

{O you who have believed, enter into Islam completely [and perfectly] and do not follow the footsteps of Satan. Indeed, he is to you a clear enemy}

This is a command from Allah the exalted to the believers, that they enter {into Islam completely} ie in all the legislation's of the religion, and that they do not abandon any of it, and that they do not be like he who took his desire as his God; if the legislated matter conformed with his desire he preforms it and if it opposed it he abandons it. Rather, the obligation is make the desire subordinate to the religion, and that he preforms all acts of goodness he is able to perform, and that which he is incapable of doing he intends doing it and remain committed to it so that he achieves it by his intention.

And hence entering into Islam completely is impossible and inconceivable without opposing the ways of the devil, He said {And do not follow the footsteps of Satan} ie by falling into disobedience of Allah. {Indeed, he is to you a clear enemy} and the clear enemy will only command evil and immorality and what harms you.

And At-Tabaree said: {completely} ie enter all of you into Islam. 

And there is no contradiction between the two explanations. For that it is reasonable to say: the command is that you all enter into all legislations of Islam. (Shaykh Rslaan)

Arabic text: 

{ 208 - 209 } { يَا أَيُّهَا الَّذِينَ آمَنُوا ادْخُلُوا فِي السِّلْمِ كَافَّةً وَلا تَتَّبِعُوا خُطُوَاتِ الشَّيْطَانِ إِنَّهُ لَكُمْ عَدُوٌّ مُبِينٌ} .
هذا أمر من الله تعالى للمؤمنين أن يدخلوا { فِي السِّلْمِ كَافَّةً } أي: في جميع شرائع الدين، ولا يتركوا منها شيئا، وأن لا يكونوا ممن اتخذ إلهه هواه، إن وافق الأمر المشروع هواه فعله، وإن خالفه، تركه، بل الواجب أن يكون الهوى، تبعا للدين، وأن يفعل كل ما يقدر عليه، من أفعال الخير، وما يعجز عنه، يلتزمه وينويه، فيدركه بنيته.
ولما كان الدخول في السلم كافة، لا يمكن ولا يتصور إلا بمخالفة طرق الشيطان قال: { وَلا تَتَّبِعُوا خُطُوَاتِ الشَّيْطَانِ } أي: في العمل بمعاصي الله { إِنَّهُ لَكُمْ عَدُوٌّ مُبِينٌ } والعدو المبين، لا يأمر إلا بالسوء والفحشاء، وما به الضرر عليكم.