As Wuukiee stated, if an apple is cut in half, crosswise, the seeds resemble a star; a five-point star. In Christian symbolism a five-point star represents the Star of Bethlehem. Five points also represents the Crucifixion.
The symbolism of an apple links the Old Testament of the Bible to the New Testament; original sin, to the birth of Jesus, to forgiveness of sins.
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The apple is also a symbol for knowledge... it's amusing that, on the first day of school, children in elementary school give their teacher an apple, and not the other way around.