The Cross is the most common symbol of Christianity and in fact was decreed by Constantine at the council of Nicaea in 325 A.D. to be the official symbol of Christianity.
From Wikipedia we read, "During the first two centuries of Christianity, the cross may have been rare in Christian iconography, as it depicts a purposefully painful and gruesome method of public persecution" but with time, the Crucifix and the Cross came to be representations of Christ's goodness to mankind. The Crucifix, which is a cross with the image of Jesus Christ on it, is a symbol of Jesus's sacrifice of suffering and ultimate death for the sins of mankind. The empty cross reminds Christians of Jesus Christ's resurrection and victory over death, along with the promise of new life and hope for mankind.
Easter is a time for Christians to reflect, remember, and rejoice about the resurrection of Jesus Christ. Thus, the cross which is a symbol of His resurrection, is also a symbol of the Easter season.
During the time of Abraham, lambs, which were a highly valued possession, were offered to God as a sacrifice for sin. The most perfect lamb from the flock was chosen as the sacrifice. The practice of sacrificing the most pure and perfect lamb for sin was a representation of the coming Savior, who, without blemish of sin, would one day sacrifice His life for mankind. John the Baptist proclaimed in John 1:29, "Behold, the Lamb of God, that taketh away the sin of the world!" With the death of Jesus Christ, the Atonement was made sure and the practice of sacrificing animals ceased.
The Lamb continues to be a symbol of Jesus Christ and his atoning sacrifice.