St. Thomas Aquinas
“The Paschal Lamb foreshadowed this sacrament in these three ways. First of all, because it was eaten with unleavened loaves, according to Ex. 12:8: ‘They shall eat flesh . . . and unleavened bread.’ As to the second because it was immolated by the entire multitude of the children of Israel on the fourteenth day of the moon; and this was a figure of the Passion of Christ, Who is called the Lamb on account of His innocence. As to the effect, because by the blood of the Paschal Lamb the children of Israel were preserved from the destroying Angel, and brought from the Egyptian captivity; and in this respect the Paschal Lamb is the chief figure of this sacrament, because it represents it in every respect.”
Source: St. Thomas Aquinas, Summa Theologica