Allah (swt) forbade allegiance to the Jews and the polytheists of Quraish, meaning that the relationship with them should not be at the expense of the values and morals that Islam brought.

(  وَاتَّقُوا اللَّهَ إِن كُنتُم مُّؤْمِنِينَ… ) (…And fear Allah, if you should [truly] be believers.)): True faith requires fearing, fearing of Allah (swt) is clarified in the following verse:

إِنَّ الْمُتَّقِينَ فِي جَنَّاتٍ وَعُيُونٍ (15) آخِذِينَ مَا آتَاهُمْ رَبُّهُمْ ۚ إِنَّهُمْ كَانُوا قَبْلَ ذَٰلِكَ مُحْسِنِينَ (16) كَانُوا قَلِيلًا مِّنَ اللَّيْلِ مَا يَهْجَعُونَ (17) وَبِالْأَسْحَارِ هُمْ يَسْتَغْفِرُونَ (18) وَفِي أَمْوَالِهِمْ حَقٌّ لِّلسَّائِلِ وَالْمَحْرُومِ

(Indeed, the righteous will be among gardens and springs, (15) Accepting what their Lord has given them. Indeed, they were before that doers of good. (16) They used to sleep but little of the night, (17) And in the hours before dawn they would ask forgiveness, (18) And from their properties was [given] the right of the [needy] petitioner and the deprived.) (Adh-Dharyat: 15-19)

And as Imam Ali (may Allah honour his face) defined it: “It is fearing of His Majesty, following the revelation and preparation for the day of departure”. Therefore, just in case you apply all the moral values and faith controls, you become a pious.