Understanding of the Religion means understanding of the Quran and hadith, religious injunctions, and knowledge of the lawful and the unlawful. 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. One has to make oneself worthy of the honour.