Catholics and the Sign of the Cross

My roomie is perplexed on the reason why do we Catholics make a sign of the cross when leaving the church. She told me that her teacher back in St. Scholastica, a Catholic school in Manila has told them that dipping your fingers to the holy water and making sign of the cross means leaving your sins there. So her question now is why again dip your hand upon leaving. Does that mean you're taking all the sins again? I just smiled back then and I don't want to give any comment on the reason I do it since I don't want to argue and it would be my words versus her teacher. I also have this respect  on teachers for I know they knew better and also simply I have not ask that before and really don't know the catechism about it. My own reason why I'm doing it is simply out of respect since it is the house of God. 

Indeed we are blessed to be in the digital age where every information is just in one click. I search in Catholic website regarding this topic and I remember back in our grade school I also have a teacher who has told us that if the host has stick to the upper portion of your mouth means you're a sinner. Then on the next day she told us to forget whatever she said. I then realized that perhaps one of my classmates might report it to her parent and the school authority learned it and reprimanded her. She then told us that she is not the right person to teach on the matters of faith. I then concluded that there are teachers who would tell their students these superstitious beliefs and we students will bring that belief up to our adolescent and will get confused. Thanks to my curiosity and eagerness to learn that I do search on reliable sources for answers. 

The Sign of the Cross - we profess the deepest mysteries of Christian Faith: The Trinity - the Father, the Son and Holy Spirit... yes I knew this but reminding me once again upon reading it gave me a big laugh. I remember too back in college I have a Protestant professor who told us not to make sign of the cross as respect for other religions. Though one of my classmates answered him back 'how about our faith?' It is ridiculous that other religion would ask for respect on their faith while trying to stop us Catholics on practicing our own. By signing ourselves we not only profess that we are Catholics but also our firm belief on the victory of Christ made on the cross and also about the Trinity.

So why make the Sign of the Cross upon entering and exiting the church?

In order to participate in the great sacrifice of the Mass, you need to be baptized. Using holy water to sign yourself is saying "I am a baptized Christian and I am authorized to participate in this sacrifice." 

When you make the sign of the cross when you leave, you say that the Mass never ends — your whole life is participating in Christ's sacrifice.  

I read a lot more about this and this simple question has lead me to many answers that enlighten me more. To summarize: The sign of the cross is: a confession of faith; a renewal of baptism; a mark of discipleship; an acceptance of suffering; a defense against the devil; and a victory over self-indulgence.