The Israelites were sinful human beings just we are. They tended to pretend that things entirely dependent upon God depended on them in some way, on their input or obedience. "If we do our part, then God will save us."
God told the Israelites to choose a lamb for the Passover. He told them exactly what the lamb should be and what it should not be. Still, God’s deliverance of the Israelites didn’t depend upon their choice of an animal. It depended on His faithfulness to His promise to deliver. The LORD didn’t promise to deliver because of some redeeming quality in the Israelites, but because of an amazing quality in HIM, namely His Grace and faithfulness. God delivered the Israelites because of who He is—faithful to His Promises, loving and merciful.
God has delivered us from our spiritual enemies and from ourselves not because there is some redeeming quality in us, but because of a quality in Him—His grace.
The LORD established the Passover for them and for future generations as an annual memorial of His deliverance. They were to continue observing this festival in order to remember God’s amazing grace and mercy. We remember Jesus’ sacrifice on Calvary because it is to us a daily (not just on Good Friday) reminder of God’s amazing grace and mercy to us.