Salvation

Philippians 2:12

So then, my beloved, obedient as you have always been, not only when I am present but all the more now when I am absent, work out your salvation with fear and trembling.

Jesus redeemed us from our sins with his suffering and death. 

But that doesn't mean our salvation is automatic. We have to do our part.

Even when our sins are forgiven -- for instance, through the sacrament of confession -- we still have to make up for the temporal punishment that is connected with sin (example: paying for a broken window).

Also, we have to live in accord with Jesus' commandments. That is why we need to pray, to sacrifice, to do acts of charity.

Think of it this way.

Imagine a young woman has a chance to go to college, but her family doesn't have the money. Along comes a rich uncle, who agrees to pay all her tuition and other expenses. 

Does that mean the young woman receives a college degree automatically? No. She has to attend class, study hard, write papers, and pass her exams. She has to do her part.

It's like that with Jesus. He "pays our tuition," but we have to attend class, pass exams, etc., before we can get a degree (salvation).