Quoting St. Thomas Aquinas, John Paul wrote:

“When through sin, the soul commits a disorder that reaches the point of turning away from its ultimate end God to which it is bound by charity, then the sin is mortal; on the other hand, whenever the disorder does not reach the point of a turning away from God, the sin is venial.’ For this reason venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity and therefore eternal happiness, whereas just such a deprivation is precisely the consequence of mortal sin” — Reconciliation and Penance, Chapter 17 (*)

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(*) See also The Catholic Encyclopedia