Consider the following quotation: "What is prayer in the fullest sense? Prayer is continuing a conversation that God has started through his Word and his grace, which eventually becomes a full encounter with him" (48).
There are several things I like about this definition. First, Keller recognizes that any communicative approach to God on our part is a response; that is, God has initiated the conversation. This speak to his graciousness. We deserve punishment, he offers his presence.
Second, the conversation was started by God through his Word and his grace. The revelation of God is seen in the created universe. It seen more clearly in his Word, the Bible and the Logos. Keller later writes "Jesus Christ is the Word of God (John 1:1-14) because no more comprehensive, personal, and beautiful communication of God is possible (49, italics in original).
Finally, Keller states this conversation "eventually becomes a full encounter with him" (48). Initially, I thought this was overstating things; I countered that not everyone does encounter him . . . some never encounter him. But really, Keller is right. I was only considering encountering God is a positive sense. But everyone WILL encounter him, some for glory and others for punishment. The conversation that began with God's Word and grace will come to a full encounter: "Well done my good and faithful servant" or "Depart from me, I never knew you."
I am left contemplating the conversation God started at with me at creation, that he continued with me at Calvary, and that he will perpetuate with me throughout eternity at the consummation of all things.