There are few images in the Celtic world more iconic than the Celtic knot. Celtic knot work came about rather suddenly in the history of the people, but it became an integral part of art, religion, and symbolism rather rapidly. There are many stories that the knot work came into Celtic history as part of the Celtic Druidic tradition, but that is not necessarily the case.
There is actually good reason to believe that Celtic Knots were a tradition that was passed on from Coptic Egypt. Old Celtic manuscripts, as well as knot work, are very similar to those found in Egypt during the height of their art, knowledge, and culture. One of the largest libraries in the ancient world existed here, so it should not be too much of a surprise that some of that knowledge made its way to what is now England, Ireland, and Scotland. A 5th century copy of the Acts of the Apostles, preserved in the Morgan Library in New York City, is seen by a number of scholars as the “missing link” between the Celtic and Middle Eastern knot work traditions.
Well, Celtic knots may have got their start in Egypt, but that start did not finally dictate the extent to which the Celtic people would develop the art of knot work. Once knots took on a “spiritual significance” with the Celtic Christians, the knots became quite complex in design. This is especially true of cross designs, and the triangle style designs used to represent Trinitarian Theology. The art developed fully by the 10 Century A.D. During the middle ages, the style spread across most of Europe.
You can find examples of Celtic knots being designed and used all the way to our current century, though the use of Celtic knots in art has fallen off a bit from the 19th Century onward. In more modern times, you see the use of the artwork in jewelry, and lately in the design of tattoos.
Modern use of Celtic knot work began to change and expand when designers like Archibald Knox and Alexander Ritchie started to make brooches in the forms of knots, and knot work became more the subject of designs, rather than taking a back seat to other messages or interests. When these modern designers began to take elements such as single knots and make these the focus of designs, or even to make a single knot the entire object, this was a profoundly creative act. Knots and details were beginning to take on lives of their own as standalone statements.