God has us keep the Passover because it forces us to consider the deaths of the firstborn Egyptians and how that miraculous and terrible event led to the freeing of Israelites from Pharaoh and from Egypt. It should lead us to think deeply about what these events symbolize.
Yet, is not Passover just an Old Testament, Jewish ritual? No! God's commands are never merely empty rituals. His commands always contain rich and meaningful purposes, including spiritual, New Testament applications that we can learn from today.
For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me." In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me." For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes. (I Corinthians 11:23-26)
Notice that this reminder to keep the Passover was recorded by the apostle Paul some years after the close of the Old Testament era. It is most decidedly a Christian observance.
He adds that our preparation for Passover should cause us to take a close look at ourselves in solemn self-examination, to see how far we have grown and how much we still need to overcome: "But let a man examine himself, and so let him eat of the bread and drink of the cup" (verse 28). In the days preceding the Passover each year, we think about the past year and how imperfect we still are, and we ask God to continue to cover our sins and imperfections with the blood of our Savior, Jesus Christ. Each of God's people makes a practice of looking back at the years that have flown by since his baptism, considering how far we have left our lives of sin behind. Self-examination shows us areas in which we still need to overcome and should motivate us to rededicate ourselves to the covenant we have made with God.