Overall meaning :
Allah, Glorified and Exalted be He, opens this honourable surah with the ‘disconnected letters’ to establish the miraculous nature of the Qur’an, which conclusively prove the Arabs’ total inability to challenge the Qur’an by producing something like it, even though it is composed of the very Arabic letters they speak!
Then He talks about His Oneness, being the only true God who alone is deserving of worship; the Ever-Living, whose eternal and perfect life necessarily entails all His other perfect attributes; the Self-Existent and the Self-Sufficient who does not does not depend on anyone or anything for His Being; the Self-Subsistent, who sustains the entire order of the uni and does not stand in need of His creatures, while all His creatures stand in need of Him. His self-subsistence and immense mercy towards His slaves is partly manifested in His sending down to His Prophet, Muhammad, may Allah bless him and grant him peace, the Qur’an, containing the truth and confirming the previous scriptures, and before it, His sending down the Torah to Moses and the Gospel to Jesus, in order to guide people to the truth. In these books He revealed the criterion whereby to distinguish truth from falsehood and right from wrong, hence His threatening the unbelievers, who defiantly reject such wonderful signs, with a severe punishment on the Day of Judgement; for indeed nothing can frustrate His plans and He is capable of taking terrible vengeance on those who disobey Him. His vengeance does not flow from His desire to exact revenge, but rather from His desire for justice.