Walyahkum ahlu al-injeeli bima anzala Allahu feehi waman lam yahkum bima anzala (surah Maidah: 47)

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(5:47) Let the followers of the Gospel judge by what Allah has revealed therein, and those who do not judge by what Allah has revealed are the transgressors. *77

Walyahkum Ahlul Injeeli bimaaa anzalal laahu feeh; wa mal lam yahkum bimaaa anzalal laahu fa ulaaa`ika humul faasiqoon

*77). Here three judgements are issued against those who do not judge in accordance with the Law revealed by God. The first is that they are kafir (unbelievers); the second, that they are zalim (wrong-doers); and the third, that they are fasiq (transgressors). This clearly means that one who, in disregard of God's commandments and of the Laws revealed by Him, pronounces judgements according to man-made laws (whether made by himself or by others) is guilty of three major offences. First, his act amounts to rejecting the commandment of God, and this rejection is equivalent to kufr (infidelity, unbelief). Second, his act is contrary to justice, for only the laws made by God are in complete accord with the dictates of justice. Any judgement in contravention of God's injunctions amounts, therefore, to committing injustice (zulm). Third, when he enforces either his own or anyone else's law in disregard of the Laws of his Lord he steps out of the fold of subjection and obedience, and this constitutes fisq (transgression).
Kufr, zulm and fisq are essential elements in deviation from God's commandments. One finds them wherever there is deviation from the commandment of God. There is variation in the degree of deviation and hence in the degree of these three offences. Whoever passes judgement on something in opposition to an injunction of God, believing that injunction to be false, and holds either his own or anyone else's judgement to be sound, is an unbeliever (kafir), wrong-doer (zalim) and transgressor (fasiq). A man who is convinced that the injunctions of God are right but makes judgements contrary to them in practice is not an unbeliever in the sense that he ceases to remain a member of the Islamic community, but he is guilty of adulterating his faith by blending it with kufr, zulm and fisq. In the same manner, those who deviate from the injunctions of God in all matters are unbelievers, wrong-doers and transgressors. For those who are obedient in some respects and disobedient in others, the blending of faith and submission to God with the opposite attributes of unbelief, wrong-doing and transgression in their lives will be exactly in proportion to the mixture of their obedience to and their deviation from God's commands.
Some commentators have attempted to restrict the application of these verses to the People of the Book alone. The verses, however, hardly lend themselves to such a restrictive interpretation. The best answer to such a restrictive interpretation has been given by the Companion Hudhayfah. When someone told him that these verses related merely to the Israelites, meaning that the unbelievers, wrong-doers and transgressors were only the Jews who passed judgement contrary to the injunctions revealed by God, Hudhayfah remarked: 'What good brothers these Israelites are to you! Whatever is bitter goes to them; whatever is sweet comes to you. Nay, by God, you will follow their way, your steps following theirs.'