Though Easter was clearly pagan in origin, Christian leaders of the first two centuries after Christ's crucifixion employed the same philosophy in establishing the new holiday that they later applied to Christmas. Believing that people are free to select their own times and customs of worship, they went about gradually replacing the biblically commanded Passover with their humanly devised celebration of Easter.
It was easier to draw pagan worshippers into Christianity and maintain their devotion by identifying the time-honored spring resurrection feast of the pagan mystery religions with the resurrection of Christ.
Anti-Jewish prejudice also seems to have been a major factor in the church leaders' decision to make such changes. According to R.K. Bishop: "The early development of the celebration of Easter and the attendant calendar disputes were largely a result of Christianity's attempt to emancipate itself from Judaism. Sunday had already replaced the Jewish sabbath early in the second century, and despite efforts in Asia Minor to maintain the Jewish passover date of 14 Nisan for Easter [or, rather, the true Passover] (hence the name Quartodecimans [meaning 'Fourteeners']), the Council of Nicaea adopted the annual Sunday following the full moon after the vernal equinox (March 21)" (Walter Elwell, editor, Evangelical Dictionary of Theology, 1984, "Easter").
Before A.D. 70, Christianity was "regarded by the Roman government and by the people at large as a branch of the Jewish religion" (Jesse Lyman Hurlbut, The Story of the Christian Church, 1954, p. 34). Christianity and Judaism shared the biblical feast days, although Christians observed them with added meanings introduced by Jesus and the apostles.
However, two Jewish revolts against the Roman Empire, in 64-70 and 132-135, led to widespread persecution of Jews and suppression of Jewish religious practices. Jews were even driven from Jerusalem and forbidden to return on pain of death. As pressure mounted, some Christians began to abandon beliefs and practices perceived as being too Jewish. Over time many abandoned their weekly Sabbath day of rest and worship in favor of worship on Sunday, the pagan day of the sun, and abandoned the Passover in favor of Easter to distance themselves from Jews.
The New Catholic Encyclopedia explains: "Originally both observances [Passover and Easter] were allowed, but gradually it was felt incongruous that Christians should celebrate Easter on a Jewish feast, and unity in celebrating the principal Christian feast was called for" (1967, Vol. 5, p. 8, "Easter Controversy").