The cross represents the ultimate human sacrifice. As you've read early in the bible, there were many animal sacrifices unto God, and Abraham almost slew his own son just to show his faith to God. Many millennia later, God decided that it was time to have a son, his only begotten son, to atone for the sins of mankind, and give one last human sacrifice, unto the cross. His son Jesus was spat on, beaten, ridiculed and finally executed and upon his death and resurrection, He was given the keys to heaven and hell, to life and death. God, whom is the beginning and the end, the principle, ineffable, unfathomable and omnipotent creator, made it clear to us all, that no sacrifice, human or animal, was needed any longer. To win his praise, we have now his son, and through Christs' death upon the cross and resurrection after, we ourselves have been shewn that there is indeed life after death, and the path to heaven is such: To know him, to love him and to serve him with all our heart.