As a symbol, the cross can have a wide variety of meanings. It represents communication between heaven and earth: the vertical line is the celestial, spiritual and intellectual, positive, active and male, while the horizontal is the earthly, passive, negative and female; the whole cross forming the primordial androgyne. It is the union of opposites and represents spiritual union and the integration of man's soul in the horizontal-vertical aspects, necessary to full life. Tree of life and tree of nourishment.
The Celtic Cross brings together the cross (male energy) and the circle (female energy) and represents fertility. Later on, in Christianity, it got to represent the union of heaven and earth.
The Rosy Cross 'echoes the Celtic cross, and its early meanings are also linked with generative power. Its more esoteric use by the rosicrucians suggested the blood of Christ spilled on the cross and the seven stages of initiation, represented by the seven petals that form the flower.' (David Fontana, The secret language of symbols)