أَفَغَيْرَ اللَّهِ أَبْتَغِى حَكَماً
(Shall I seek a judge other than Allah...) between you and I,
وَهُوَ الَّذِى أَنَزَلَ إِلَيْكُمُ الْكِتَـبَ مُفَصَّلاً
(while it is He Who has sent down unto you the Book, explained...) in detail,
وَالَّذِينَ ءَاتَيْنَـهُمُ الْكِتَـبَ
(and those unto whom We gave the Scripture) the Jews and the Christians,
يَعْلَمُونَ أَنَّهُ مُنَزَّلٌ مِّن رَّبِّكَ بِالْحَقِّ
(know that it is revealed from your Lord in truth.) because the previous Prophets have conveyed the good news of you coming to them. Allah's statement,
فَلاَ تَكُونَنَّ مِنَ الْمُمْتَرِينَ
(So be not you of those who doubt.) is similar to His other statement,
فَإِن كُنتَ فِي شَكٍّ مِّمَّآ أَنزَلْنَآ إِلَيْكَ فَاسْأَلِ الَّذِينَ يَقْرَءُونَ الْكِتَـبَ مِن قَبْلِكَ لَقَدْ جَآءَكَ الْحَقُّ مِن رَّبِّكَ فَلاَ تَكُونَنَّ مِنَ الْمُمْتَرِينَ
(So if you are in doubt concerning that which We have revealed unto you, then ask those who are reading the Book before you. Verily, the truth has come to you from your Lord. So be not of those who doubt (it).) 10:94 The conditional `if' in this Ayah does not mean that `doubt' will ever occur to the Prophet . Allah said,
وَتَمَّتْ كَلِمَةُ رَبِّكَ صِدْقاً وَعَدْلاً
(And the Word of your Lord has been fulfilled in truth and in justice.) Qatadah commented, "In truth concerning what He stated and in justice concerning what He decided." Surely, whatever Allah says is the truth and He is Most Just in what He commands. All of Allah's statements are true, there is no doubt or cause for speculation about this fact, and all His commandments are pure justice, besides which there is no justice. All that He forbade is evil, for He only forbids what brings about evil consequences. Allah said in another Ayah,
يَأْمُرُهُم بِالْمَعْرُوفِ وَيَنْهَـهُمْ عَنِ الْمُنْكَرِ
(He commands them with good; and forbids them from evil...) 7:157 until the end of the Ayah.
لاَ مُبَدِّلَ لِكَلِمَـتِهِ
(None can change His Words.) meaning, none can avert Allah's judgment whether in this life or the Hereafter,
وَهُوَ السَّمِيعُ
(And He is the All-Hearer,) Hearing, His servants' statements,
الْعَلِيمُ
(The All-Knower.) of their activities and lack of activity, Who awards each according to their deeds.
Tafsir Ibn Kathir (abridged, English).