Tackling this question is a little like trying to transfer the ocean into a small hole on the beach, to borrow an image from St. Augustine.
As Catholics we believe we reach heaven through God’s grace but also that we have to cooperate with that grace. St. Paul writes that we “work out [our] salvation with fear and trembling.” (Phil. 2:12) This and other scriptures are interpreted differently, depending on who’s taking up the question. My goal with this post is simply to present very basic Catholic teaching on the subject.
Getting to heaven is about grace, which we can’t earn and which comes from the love and mercy of God. According to the Catechism, “The grace of the Holy Spirit has the power to justify us, that is, to cleanse us from our sins and to communicate to us ‘the righteousness of God through faith in Jesus Christ’ and through Baptism.” (CCC 1987)
Grace makes salvation from sin possible because we believe in God’s revelation and promises, fear God’s judgment, hope in His mercy, trust that God will be merciful to us for Christ’s sake, begin to love God as the source of justice and detest our sins.
The grace to respond
We do have to do something, though. The Catechism says we must give our free response to the gift of grace, even though we need grace just to respond, as the Council of Trent concluded:
“…whereby, without any merits existing on their parts, they are called; that so they, who by sins were alienated from God, may be disposed through His quickening and assisting grace, to convert themselves to their own justification, by freely assenting to and co-operating with that said grace: in such sort that, while God touches the heart of man by the illumination of the Holy Ghost, neither is man himself utterly without doing anything while he receives that inspiration, forasmuch as he is also able to reject it; yet is he not able, by his own free will, without the grace of God, to move himself unto justice in His sight.”
Along with making a response to grace through faith, Scripture tells us that in order to be saved, we must be baptized (Mk. 16:16), we must receive Christ’s true body and blood (John 6:54) and we must obey the commandments (Matt. 19:17 The sacraments are visible assurances that God is providing us with the grace to keep going.
Our call as Christians is to the “fullness of Christian life and to the perfection of charity. All are called to holiness: ‘Be perfect, as your heavenly Father is perfect.’” (CCC 2013)
In short, we have to work at becoming holy. The good works that spring from God’s grace are evidence that we’re cooperating with that grace. Faith alone won’t save us; we have to persevere in doing good, as Christ said in his description of final judgment in Matt. 25:31-46. In this parable, he calls us to charity: “Whatever you did for one of these least brothers of mine, you did for me.”
St. Augustine sums it up well:
Indeed we also work, but we are only collaborating with God who works, for His mercy has gone before us. It has gone before us so that we may be healed, and follows us so that once healed, we may be given life; it goes before us so that we may be called, and follows us so that we may be glorified; it goes before us so that we may live devoutly, and follows us so that we may always live with God: for without Him we can do nothing. (CCC 2001)
As far as I can tell, there is no stairway to heaven—nor is there an escalator. The Catholic Church teaches that we get there by God’s grace, as well as by the works we do through that grace. Established by Christ, she is our best guide.
Salvation comes from God alone; but because we receive the life of faith through the Church, she is our mother: “We believe the Church as the mother of our new birth, and not in the Church as if she were the author of our salvation. Because she is our mother, she is also our teacher in the faith.” (CCC 169)