The Catechism in Questions & Answers

546. Can we help the departed attain salvation?

Yes, we can intercede in prayer for unredeemed souls and ask the Lord to help them. Likewise, we can pray that these souls come to believe in Jesus Christ and that they are open and prepared to accept the salvation God wishes to grant them.
Since both the living and the dead in Christ comprise a single fellowship, they will work both here and in the beyond in the mind of Christ, in other words, they will intercede in prayer for the unredeemed.
Redemption itself can only occur through Jesus Christ, however.

Redemption: see Questions 215. et seq. Salvation: see Questions 243., 248. et seq.