Ever since Levin had seen his brother dying, the questions of death and life have weighed heavy on his mind.
Although he has continued living, Levin has been terrified of his own ignorance.
Marriage temporarily distracted him from these bigger questions, but ever since his time in Moscow, his brain has been occupied with questions of the mysteries of life. To this end he has been reading lots of philosophy.
One thing Levin has realized is that everyone close to him is a believer. After reading a lot of books, Levin fails to be persuaded by even those who ostensibly share his same disbelief.
Levin remains concerned about his moment of prayer during Kitty's pregnancy.