How does the Catholic Church reconcile Catechism #841 which says, "The plan of salvation also includes Muslims" (who deny the resurrection) with 1 Corinthians 15:14 that says "If Christ be not risen...your faith is in vain."?
God surely wants to save all and has set plans in place to do so through the preaching of the Gospel. That the plan of salvation aims for all does not mean that all are in fact saved. If one were to knowingly reject Christ after having Him effectively preached, they may well forfeit their salvation. However, not all have had Christ effectively preached, and the Church leaves the final determination to the Lord of how culpable they are of their seeming rejection of Christ.
A short biblical answer to your question is supplied by Peter in Acts. Having heard a sermon that he preached on Pentecost, many were struck to the heart and cried out what shall we do? Peter replied, repent and be baptized every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. (Acts 2:38)
But this is not simply to be understood as a ritualistic observance that we fulfill on one day, but is meant to usher in a whole renewal of the human person. And thus, we ought to look at all three things that Peter indicates in some more detail.
The word translated as “repent” is Metanoia which means more than to clean up our act. It means to come to a whole new mind, rooted in what God teaches and reveals, with new priorities and able to make better decisions.
To be baptized is not only to be cleansed of our sins but also to see our old self put to death, and for Christ to come alive in us. Baptism ushers in the beginnings of a lifelong healing process that must continue by God's grace. Baptism also points to all the sacraments of the Church. For, having been brought to new life, we must also be fed by the Eucharist and by God's word, we must see the wounds of sins healed in confession, we must be strengthened for a mission by confirmation. Baptism also makes us a member of the body of Christ. And thus, we are called to walk in fellowship with all the members of Christ’s one body, The Church.
St. Peter also speaks of receiving the gift of the Holy Spirit. And thus, we are taught that our dignity is to be swept up into the life, love, and wisdom of God. We are called to be sanctified by the Holy Spirit to see sins put to death and many virtues come alive.
Finally, something needs to be said about your use of the word "exactly" which might imply there is some very simple formula for getting saved. But as can be seen, there are many dimensions to the work of God in saving us. Thus, we are to walk in a loving covenant relationship with the Lord and His body the Church. And like any relationship, this cannot simply be reduced to a few things. We must trust the Lord and walk in a relationship of love and obedience to him. We are to do this in fellowship with His Church, through the grace of the Sacraments, obedience to the Word of God, and prayer (cf Acts 2:42).
"Building our Catholic faith one question at a time."