Allah's statement,

﴿وَإِنْ أَطَعْتُمُوهُمْ إِنَّكُمْ لَمُشْرِكُونَ﴾

(and if you obey them, then you would indeed be polytheists.) means, when you turn away from Allah's command and Legislation to the saying of anyone else, preferring other than what Allah has said, then this constitutes Shirk. Allah said in another Ayah,

﴿اتَّخَذُواْ أَحْبَـرَهُمْ وَرُهْبَـنَهُمْ أَرْبَاباً مِّن دُونِ اللَّهِ﴾

(They (Jews and Christians) took their rabbis and their monks to be their lords besides Allah.)﴿9:31﴾ In explanation of this Ayah, At-Tirmidhi recorded that `Adi bin Hatim said, "O Allah's Messenger! They did not worship them.'' The Prophet said,

«بَلَى إِنَّهُمْ أَحَلُّوا لَهُمُ الْحَرَامَ وَحَرَّمُوا عَلَيْهِمُ الْحَلَالَ فَاتَّبعُوهُمْ فَذَلِكَ عِبَادَتُهُمْ إِيَّاهُم»

(Yes they did. They (monks and rabbis) allowed the impermissible for them and they prohibited the lawful for them, and they followed them in that. That was their worship of them.)

﴿أَوَمَن كَانَ مَيْتًا فَأَحْيَيْنَـهُ وَجَعَلْنَا لَهُ نُورًا يَمْشِي بِهِ فِى النَّاسِ كَمَن مَّثَلُهُ فِي الظُّلُمَـتِ لَيْسَ بِخَارِجٍ مِّنْهَا كَذَلِكَ زُيِّنَ لِلْكَـفِرِينَ مَا كَانُواْ يَعْمَلُونَ ﴾

(122. Is he who was dead, and We gave him life, and set for him a light, whereby he can walk among men -- like him who is in the darkness from which he can never come out Thus it is made fair seeming to the disbelievers that which they used to do.)