I believe this is one of those "hot buttons" of theology that many Christians get hung up on. Now we know that many Christians label themselves as one, but if you were to travel through out a week in their shoes you'd see they use the term loosely. And we would probably come to that conclusion--- though I'm personally wary of judging other believers, unless it is for their benefit of learning from their sins.
Anyways, the Catholic Church holds this belief:
In sum, the Church teaches that salvation is a process of becoming holier and holier through time. All of this is a work of grace that God performs in our hearts through faith. Works done in faith are the natural completion of believing in Christ. As we trust and do God’s work, he instills within us more grace so that we may become holier and so be ready to meet him at the end of our life.
(Quote from Catholic.com)
Salvation comes from the grace of God because God the Son, Jesus Christ died on the Cross. But each of our faith stories does not end there: we grow as a Christian-- we become holier and holier the more we allow the grace of God to work in us and through us.
It's like a plant--- it comes from a small seed, but in order to get the fruit from that plant it takes many resources, it takes time, and the plant itself must transform from a tiny seed to baby leaves and stem, to stronger roots, bigger leaves, and finally then the fruit will be there.
So too must we each reflect on our own lives-- we need to make sure our own faith is not stagnant. We have salvation through Christ, but His ministry continues through each of us and we only grow in holiness if we're living the faith, the morals, and the love Christ lived through as a human being on earth.