The Midrash says that in the next world Hashem will teach the Jewish people Torah, and only those who learned that portion in this world will understand His teachings. Surely this doesn’t refer to people who just read the words in this world, because that is not considered learning. Rather it means that if a person does his best to understand what our Sages wanted to teach us, then in the next world Hashem will teach him the inner meanings of what he learned that he could not possibly fathom in this world. A person who understands this will strive to understand what he learns deeply, to merit being taught by Hashem the full depth of the Torah, for what he is negligent in trying to understand he will not be taught.
כ״ז באייר ה׳תש״פ
"והאמת בלתי שום ספק כלל, שאם היה העולם כולו מקצה עד קצהו פנוי חס ושלום אף רגע אחד ממש מהעסק והתבוננות שלנו בתורה, כרגע היו נחרבים כל העולמות, עליונים ותחתונים, והיו לאפס ותוהו חס ושלום". (נפש החיים) 1