إِنَّ ٱلَّذِينَ قَالُوا رَبُّنَا ٱللَّهُ ثُمَّ ٱسْتَقَـٰمُوا فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ

One argument in favour of the veracity of the Quran is that the previous revealed scriptures predicted it. These predictions are still found in the Bible (Injeel) and the Torah. The Quran thus came as a realisation of earlier divine predictions, giving concrete shape to what had been foreseen many centuries before. This is a clear indication that the Quran is really a divine Book; otherwise how could it have been possible to give advance information about it hundreds and thousands of years ago? It is reported on the authority of ‘Abdullah ibn ‘Abbas that verse 13, broadly interpreted, means being steadfast in the discharge of duties attached to Faith. (Tafsir ibn Kathir). Faith (iman) is a sacred pledge or vow. Time and again, moments of trial occur in the life of a man when he either keeps his pledge of Faith, or he breaks it. On such occasions, one who acts in accordance with his pledge of Faith, shows his steadfastness, while one who fails to do so, shows his lack of devotion. Those unable to prove their steadfastness are transgressors, while those who prove to be steadfast are the ones who will be lodged in the eternal gardens of Paradise.

— Source: Tazkirul Quran (Maulana Wahiduddin Khan) via Quran.com (46:13)