Today's Gospel reading answers the question: "Can people lose their salvation?" Many Protestants believe in the theology of "once saved always saved" and that heaven is forever guaranteed on the day of conversion when a person says yes to the redemptive death and resurrection of Jesus.
Catholic teaching, however, acknowledges that a conversion might not be sincere or complete, and that deliberate, terrible sins will turn an unrepentant sinner away from Christ forever (we call such sins "mortal", because they kill the soul).
Knowing this can happen, many good Catholics fear that someday they might choose to turn away from Christ.
Salvation is more than knowing who Jesus is. Many know who he is without knowing him. He is more than a who. Demons know who he is and even obey his commands. Obedience alone does not get anyone into heaven.
Jesus is more than an authority we must obey. To know Jesus is to know what he's all about (his purpose, his love, and his life). When we honestly choose to trust in the "what" about Jesus, we naturally want to be just like him. We want to follow him, doing what he does, all the way to heaven.
People can believe in Jesus and yet remain in the darkness of sin and eternal death. To live in the light of Christ and remain there, we must not only believe that he is God. We must not only believe that he is Savior. We must also believe in everything -- oh yes, everything! -- that he taught by word and by deed.
We birth Jesus into the world (guiding others toward heaven) and we ensure our own place in heaven by listening to his words and acting upon them. Salvation is more than our words of belief. It's our actions, which we do because we believe. We act the way Jesus acted and we do what Jesus did, and this is how Jesus is born again and again from us -- through our behavior.
Today's Gospel reading ends the Sermon on the Mount, which started with Matthew's fifth chapter. Read this entire sermon to find out how well you are sharing Christ with the people around you. We birth Christ into the world, for example, when we love our enemies, or when we forgive, or (as he said earlier in this sermon) when we do more than what is asked of us by going the extra mile.
Obedience is merely the minimum. To carry Christ into the world, we have to embrace the way he carried the cross, caring so much for others that we gladly make sacrifices. And although the cross looks like the antithesis of Christmas, it is the reason that Christmas exists. Isn't this why Christmas is the biggest gift-giving holiday of the year?
Love is what motivates us to do more than the minimum. If we love, we cannot help but want to do more, because unconditional, sacrificial love is the nature of Jesus. When we love others, he is loving them through us, and thus we are united to him and will most certainly go to heaven with him.