The Messenger of Allah (peace be upon him) said,
“When Allah wishes good for someone, He bestows upon him the understanding of the Deen.” [Hadith, Bukhari]
Explanation: Understanding the religion (Deen) means understanding the Quran and Hadith as well as religious injunctions and having knowledge of the lawful and unlawful prescribed upon us.
True understanding necessarily generates action that reflects the understanding. For example, most people have not personally experienced the devastation caused by a nuclear weapon, but do you have any doubt that they will do all that is in their power not to have one dropped on them?
Thus, when understanding of the Deen exists, it will result in righteous action and bring one on to the path leading to Paradise. This will ensure everlasting happiness for that person, without a single blemish to mar it.
The blessing of understanding Islam, such that one's belief and behaviour reflect it, is a sign of Allah's special favour to that person. You have to make oneself worthy of the honour :)