Let's do some basic math....we're talking basic; 1^st grade stuff.

1+1=2

2+2=4

How about this one:

1+1+1=1

The answer to that last problem is 3.  However, theologically speaking, the answer is one.  ONE?!  That's right-one!  This simple math formula isn't as simple as it looks.  This is a formula for helping to understand the Trinity: Three persons in one God.

1+1+1=1

Father+Son=Holy Spirit=God

Every time we cross ourselves, we do so in the name of the Father and of the Son and of the Holy Spirit.  Yet, it can be such a difficult thing for our minds to grasp. 

In trying to better understand the Trinity, it might be a good idea to start in Scripture.  Not in the New Testament, like you might expect, but in the Old Testament.  The Trinity is in the Old Testament?  Yes, it is, and it's right there from the beginning. 

The earth was without form and void, and darkness was upon the face of the deep; and the Spirit of God was moving over the face of the waters. (Genesis 1:2)

Later, in the same chapter, we see a reference to the Trinity during the creation of man:

Then God said, "Let US make man in OUR image, after OUR likeness." (Genesis 1:26)

The Trinity acted together in creation because all three persons existed together from the beginning of time. 

This is just an introduction.  There is so much to look at.  Next time, we'll stay in the Book of Genesis, looking at the story of the Hospitality of Abraham, which gives us our icon of the Holy Trinity.