The ankh is a cross with a looped top which, besides the concept of life, also symbolized eternal life, the morning sun, the male and female principles, the heavens, and the earth. Its form embodied these concepts in its key-like shape; in carrying the ankh, one was holding the key to the secrets of existence. The union of opposites (male and female, earth and heaven) and the extension of earthly life to eternal, time to eternity, were all represented in the form of the looped cross. The symbol was so potent, and so long-lived in Egyptian culture (dating from the Early Dynastic Period in Egypt, c. 3150-c. 2613 BCE), that it is no surprise it was appropriated by the Christian faith in the 4th century CE as a symbol for their god.
The origin of the ankh symbol is unknown, but Egyptologist E. A. Wallis Budge claims it may have developed from the tjet, the ‘Knot of Isis,’ a similar symbol with the arms at its sides associated with the goddess. Female deities were as popular, and seem to be considered more powerful (as in the example of the goddess Neith), in the early history of Egypt, and perhaps the ankh did develop from the tjet, but this theory is not universally accepted.
The ankh was closely associated with the cult of Isis, however, and as her popularity grew, so did that of the symbol. Many different gods are depicted holding the ankh and it appears, along with the djed symbol, in virtually every kind of Egyptian artwork from sarcophagi to tomb paintings, palace adornments, statuary, and inscriptions. As an amulet, the ankh was almost as popular as the scarab and the djed.