(while he is a Muhsin) i.e., doing what his Lord has commanded and abstaining from what He has forbidden,
(then he has grasped the most trustworthy handhold.) means, he has received a firm promise from Allah that He will not punish him.
(And to Allah return all matters for decision. And whoever disbelieves, let not his disbelief grieve you.) means, `do not grieve over them, O Muhammad, because they disbelieve in Allah and in the Message you have brought, for their return will be to Allah and He will tell them what they used to do,’ i.e., He will punish them for it.
(Verily, Allah is the All-Knower of what is in the breasts.) and nothing whatsoever is hidden from Him. Then Allah says:
(We let them enjoy for a little while,) means, in this world,
(then in the end We shall oblige them) means, `We shall cause them,’
(to (enter) a great torment.) means, a torment that is terrifying and difficult to bear. This is like the Ayah,
(“Verily, those who invent a lie against Allah, will never be successful.” Enjoyment in this world! and then unto Us will be their return, then We shall make them taste the severest torment because they used to disbelieve.) (10:69-70)