The moon is a feminine symbol. It is associated with water, due to the gravitational influence of the moon on the ocean tides. The visible phases of the moon take place over a 28 day cycle, as does the human menstrual cycle. The moon therefore reminds us of the cyclical nature of the universe, where there is activity followed by rest, followed by activity, and so on.
The moon in its waxing phase represents growth, since when the moon is waxing it is growing larger, growing toward fullness. It is also associated with youthfulness, and in ancient pagan cultures represented the goddess in her virginal, maiden aspect. It is perhaps due to this goddess association in paganism that the waxing moon became a symbol of the Virgin Mary. The moon is used in connection with an apocalyptic image in Revelation 12:1 depicting "a woman clothed with the sun, with the moon under her feet…" The female subject of this image is interpreted by some as representing the Virgin Mary.
The moon reflects the light of the sun. It therefore serves as a reminder to Christians that, as the moon reflects the light of the sun (s-u-n), so should the Christian reflect the light of the Son (s-o-n), which reflection, like the waxing moon, should be perpetually growing in fullness and intensity within us.