In Sacred Scripture, The Epistle of James tells us that, "The prayer of a righteous man has great power in its effects." (5:16) We therefore implore those in the heavenly host to pray for us to the Lord our God, as we would ask our brothers and sisters on Earth to pray for us. To learn more about this topic, here's an article that answers many of the common questions.
One approach to this can be taken through the lens of baptism as the sign of entry into the New Covenant. In the Old Covenant, entry was marked by circumcision-- which normally occurred during infancy. Saint Paul draws a parallel between baptism and circumcision in his Epistle to the Colossians when he says, "In him also you were circumcised with a circumcision made without hands, by putting off the body of flesh in the circumcision of Christ..." (2:11) You can read more about infant baptism here.
The Catechism of the Catholic Church, which I approved 25 June last and the publication of which I today order by virtue of my Apostolic Authority, is a statement of the Church's faith and of Catholic doctrine, attested to or illumined by Sacred Scripture, Apostolic Tradition and the Church's Magisterium. I declare it to be a valid and legitimate instrument for ecclesial communion and a sure norm for teaching the faith.
— Pope St. John Paul II, Fidei depositum, part IV