Prayer is the spiritual offering that has replaced the ancient sacrifices. What good do I receive from the multiplicity of your sacrifices? asks God. We learn from the gospel: The hour will come when true worshipers will worship the Father in spirit and in truth. God is spirit, and so he looks for worshipers who are like himself.
We are the true worshipers and the true priests. Praying in spirit we offer prayer to God as a sacrifice. Prayer is an appropriate and an acceptable sacrifice to God. It is the offering he has asked for and the offering he expects.
We must make this offering with our whole heart. We must fatten it on faith, prepare it by truth, keep it unblemished by innocence, spotless by chastity, and we must crown it with love. We must escort it to the altar of God in a procession of good works to the sound of psalms and hymns. Then it will gain for us all that we ask of God. What can God refuse to prayer offered in spirit and in truth, when he himself asks for such prayer? How many proofs of its efficacy we read about, hear of, and believe!
-- Tertullian of Carthage, De oratione 28-29: CCL 1, 273-274