Allah informs all people that a plain, unequivocal proof has come to them from Him. One that eradicates all possibility of having an excuse, or falling prey to evil doubts. Allah said,
(and We sent down to you a manifest light.) that directs to the Truth. Ibn Jurayj and others said, "It is the Qur'an."
(So, as for those who believed in Allah and held fast to depend on Him,) by worshipping Him and relying on Him for each and every thing. Ibn Jurayj said that this part of the Ayah means, "They believe in Allah and hold fast to the Qur'an."
(He will admit them to His mercy and grace,) meaning, He will grant them His mercy and admit them into Paradise, and will increase and multiply their rewards and their ranks, as a favor and bounty from Him.
(and guide them to Himself by a straight path.) and a clear way that has no wickedness in it or deviation. This, indeed, is the description of the believers in this life and the Hereafter, as they are on the straight and safe path in matters of action and creed. In the Hereafter, they are on the straight path of Allah that leads to the gardens of Paradise.