In The Name of Allaah, The Most Merciful, The Bestower of Mercy

Allaah [The Most High] said in Surah Aal Imraan Ayah 100:

يَا أَيُّهَا الَّذِينَ آمَنُوا إِنْ تُطِيعُوا فَرِيقًا مِنَ الَّذِينَ أُوتُوا الْكِتَابَ يَرُدُّوكُمْ بَعْدَ إِيمَانِكُمْ كَافِرِينَ

O you who believe! If you obey a group of those who were given the Scripture, they would [indeed] render you disbelievers after you have believed!

Few Reminders from the Above Aayah:

That is because of their envy and hatred towards you [believers], and their extreme eagerness to turn you away from your religion, just as He (The Most High) said:

 وَدَّ كَثِيرٌ مِنْ أَهْلِ الْكِتَابِ لَوْ يَرُدُّونَكُمْ مِنْ بَعْدِ إِيمَانِكُمْ كُفَّارًا حَسَدًا مِنْ عِنْدِ أَنْفُسِهِمْ مِنْ بَعْدِ مَا تَبَيَّنَ لَهُمُ الْحَقُّ

Many of the people of the Scripture wish that if they could turn you away as disbelievers after you have believed, out of envy from their ownselves, even, after the truth (that Muhammad Peace be upon him is Allah’s Messenger) has become manifest unto them. [2:109] [Ref 1]

Az-Zujaaj [rahimahullaah] said: If you obey and blindly follow them [they would return you to disbelief] [Ref 2]

NB: The above has nothing to do with having peace treaties with non-Muslims, fulling contracts and behaving oneself whilst living in non-Muslim countries. See articles on this links

http://www.abukhadeejah.com/treaties-with-the-non-muslims-do-they-necessitate-allegiance/

http://www.abukhadeejah.com/how-to-conduct-oneself-with-the-jews-christians-others-even-if-they-be-tyrants/

http://www.salafipublications.com/sps/sp.cfm?subsecID=LSC01&articleID=LSC010001&articlePages=1

[Ref 1: Source: (Tayseer al-Kareem Ar-Rahmaan Fee Tafseer Kalaam Al-Mannaan). Abridged & slightly paraphrased]

[Ref2: Source: (Zaadul Maseer’ by Imaam Ibnul Jawzi). Sliglyly paraphrased]