Such questions merit more than just a quick answer. When I was asked that question, unprepared, the first thing that came to my mind was faith.

Because that's what Scripture would say after all, for example:

“for God is one and will justify the circumcised on the basis of faith and the uncircumcised through faith.” (Rom 3:30)

“It was not through the law that the promise was made to Abraham and his descendants that he would inherit the world, but through the righteousness that comes from faith.” (Rom 4:13)

“Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ,” (Rom 5:1)

But I want to explain some things further, because I do not want to have this confused with the notion of 'faith alone'. That's a different thing. In fact, the only time the words 'faith' and 'alone' are together in the Bible (even in the Greek), it is to actually warn us against 'faith alone':

“See how a person is justified by works and not by faith alone [οὐκ ἐκ πίστεως μόνον].” (Jas 2:24)

And when we look for the a similar concept 'faith only', at least in English, it leads us to a key characteristic of the faith that saves:

“For in Christ Jesus neither circumcision nor uncircumcision counts for anything, but only faith working through love.” (Gal 5:6)

So, faith, yes of course; but it has to be an operative faith, a faith that is shown in deeds. That's what St. James would insist on in his letter (cf. Jas 2:14-26). Deeds on their own could not bring us to God: that would be repeating the tragic story of the Tower of Babel in our own lives.

What is necessary is an operative, living faith, a faith that is lived out in the daily circumstances of our lives. And as Jesus said, “You will recognize them by their fruits.” (Matt 7:16) If faith is truly present in our lives, it should manifest in how we live, how we see the circumstances in our lives and in the world, how we deal with the people around us.

Faith has to bear fruit. In the Breviary, the Church often reminds us to meditate on these words of St. Paul (Rom 5:1–2, 5), where we can see how the three theological virtues -faith, hope, and charity- are intertwined (the first verse was already considered above, at the start):

“Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ. Through him we have also obtained access by faith into this grace in which we stand, and we rejoice in hope of the glory of God. [...] and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us.”

Also another important fruit of faith mentioned here is peace (which is so much part of showing our faith in deeds), also one of the fruits of the Holy Spirit. After all: “Blessed are the peacemakers, for they shall be called sons of God.” (Matt 5:9)

Let's go back to St. Paul; among faith, hope, and love, for him faith is only the start; a necessary condition, but the start, not the end. Faith has to lead to its greatest fruit, that abides forever.

“So now faith, hope, and love abide, these three; but the greatest of these is love.” (1 Cor 13:13)

The greatest of these is love...Because after all, “Anyone who does not love does not know God, because God is love.” (1 John 4:8) Like how we hear it in homilies: to be CHRISTIAN is to have Christ. But if we don't have love, we don't have Christ; and without Christ the CHRISTIAN is reduced to I.A.N.: "I am nothing."

And so not faith alone: it has to be faith that grows together with hope and charity. And all this is the ever-abiding fruit that Jesus wants us to bear in our lives:

“You did not choose me, but I chose you and appointed you that you should go and bear fruit and that your fruit should abide...” (John 15:16)

So, faith that leads to love. And that is how we understand the same question that someone asked Jesus:

“And as he was setting out on his journey, a man ran up and knelt before him and asked him, “Good Teacher, what must I do to inherit eternal life?” (Mark 10:17)

And our Lord's reply was this: “You know the commandments: ‘Do not commit adultery, Do not murder, Do not steal, Do not bear false witness, Honor your father and mother.’ ...” (Luke 18:20)

So, it's the commandments after all? Not faith, nor love? So I just "follow the rules", then I'm set for heaven? And St. Paul would have to answer: from that point of view, no. If our fulfillment of the commandments does not lead us to grow in love for God and neighbor, we would have missed the point by a mile.

“For the commandments, “You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,” and any other commandment, are summed up in this word: “You shall love your neighbor as yourself.” Love does no wrong to a neighbor; therefore love is the fulfilling of the law.” (Rom 13:9–10)

Jesus calls us to go and bear fruit, with our lives rooted in faith. Rooted in the divine life that God gave us in Christ, through our Baptism. The life of God; and God is love.

Faith is like a planted seed. It shouldn't just stay a seed. It grows, it bears fruit ... because it is a living faith. And that is real faith: living faith in the living God, who leads us to salvation.

Faith cannot be treated like a magic amulet, that I just "keep it in my pocket" and that's it. A "faith" like that is what our Lord warns us about:

“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven.” (Matt 7:21)

The Faith that is lived and that bears fruit should be the solid foundation of our Christian lives:

“Everyone who comes to me and hears my words and does them, I will show you what he is like: he is like a man building a house, who dug deep and laid the foundation on the rock. And when a flood arose, the stream broke against that house and could not shake it, because it had been well built.” (Luke 6:47–48)