The Messenger of Allah recited this Ayah;
[اتَّخَذُواْ أَحْبَـرَهُمْ وَرُهْبَـنَهُمْ أَرْبَاباً مِّن دُونِ اللَّهِ]
(
They took their rabbis and their monks to be their lords besides Allah). `Adi commented, "I said, `They did not worship them.''' The Prophet said,
«بَلَى إِنَّهُمْ حَرَّمُوا عَلَيْهِمُ الْحَلَالَ وَأَحَلُّوا لَهُمُ الْحَرَامَ فَاتَّبَعُوهُمْ فَذَلِكَ عِبَادَتُهُمْ إِيَّاهُم»
(
Yes they did. They (rabbis and monks) prohibited the allowed for them (Christians and Jews) and allowed the prohibited, and they obeyed them. This is how they worshipped them.) The Messenger of Allah said to `Adi,
«يَا عَدِيُّ مَا تَقُولُ؟ أَيُفِرُّكَ أَنْ يُقَالَ: اللهُ أَكْبَرَ؟ فَهَلْ تَعْلَمُ شَيْئًا أَكْبَرَ مِنَ اللهِ؟ مَا يُفِرُّكَ؟ أَيُفِرُّكَ أَنْ يُقَالَ: لَا إِلَهَ إِلَّا اللهُ؟ فَهَلْ تَعْلَمُ مَنْ إِلهٌ إِلَّا اللهُ؟»
(O `Adi what do you say Did you run away (to Ash-Sham) so that 'Allahu Akbar' (Allah is the Great) is not pronounced Do you know of anything greater than Allah What made you run away Did you run away so that `La ilaha illallah' is not pronounced Do you know of any deity worthy of worship except Allah)
The Messenger invited `Adi to embrace Islam, and he embraced Islam and pronounced the Testimony of Truth. The face of the Messenger of Allah beamed with pleasure and he said to `Adi,
«إِنَّ الْيَهُودَ مَغْضُوبٌ عَلَيْهِمْ وَالنَّصَارَى ضَالُّون»
(Verily, the Jews have earned the anger (of Allah) and the Christians are misguided.) Hudhayfah bin Al-Yaman, `Abdullah bin `Abbas and several others said about the explanation of,
[اتَّخَذُواْ أَحْبَـرَهُمْ وَرُهْبَـنَهُمْ أَرْبَاباً مِّن دُونِ اللَّهِ]
(They took their rabbis and their monks to be their lords besides Allah...) that the Christians and Jews obeyed their monks and rabbis in whatever they allowed or prohibited for them. This is why Allah said,
[وَمَآ أُمِرُواْ إِلاَّ لِيَعْبُدُواْ إِلَـهاً وَحِداً]
(while they were commanded to worship none but One God), Who, whatever He renders prohibited is the prohibited, whatever He allowed is the allowed, whatever He legislates, is to be the law followed, and whatever He decides is to be adhered to;
[لاَّ إِلَـهَ إِلاَّ هُوَ سُبْحَـنَهُ عَمَّا يُشْرِكُونَ]
(None has the right to be worshipped but He. Hallowed be He above what they associate (with Him).) Meaning, exalted, sanctified, hallowed above partners, equals, aids, rivals or children, there is no deity or Lord worthy of worship except Him.
http://tafsir.com/default.asp?sid=9&tid=20998