The Trinity Knot (also known as the Triquetra) is a Celtic symbol dating back to about 600 AD and is used often in Celtic mythology. It is pagan in origin and initially representing the three symbols of femininity (maiden, the mother, and the crone). After the Celts converted to Christianity, however, it came to be known as a symbol for the Father, the Son, and the Holy Spirit. Today, the Trinity Knot frequently appears on jewelry. It is a very simple knot to tie.
Hold the ribbon in your left hand, and create a loop by overlaying the right side of the ribbon over top of the left. Repeat this twice so that you have three loops of approximately the same size. Leave no extra ribbon in between the three loops. But there should be spare ribbon hanging on either end of the three-loop cluster. The spare ribbon on the left is Side A. The spare ribbon on the right is called Side B.
Without letting the loops unravel, lift Side A over Side B, and pull it up through the right loop. Pull Side B through the left loop.
Place Side A beneath the middle loop. Pull Side B down through the middle loop on the left side of Side A. Then pull Side B up through the middle loop on the right side of Side A. Side A and Side B should now cross each other in the middle loop and extend upwards. This is the Celtic Trinity Knot.