The cross is most well known in its Christian context as a symbol of sacrifice and salvation in reference to Jesus's death by crucifixion upon a cross. However, the cross has numerous other religious meanings as well, most of which have to do with being groups of four, corresponding to the four points on the cross.
Crosses very commonly represent the earth and the physical universe, particularly in Western culture. This comes primarily from two associations: the four physical elements (earth, water, air and fire) and the four cardinal directions (north, south, east and west). Indeed, the astrological symbol for Earth is a cross inside a circle. The same symbol is also known as a sun cross or solar wheel and has been associated with the sun and its four seasons.
Crosses share many properties with squares, which tend to have even more material meanings than crosses.