Prayer does not have to mean talking to an invisible being.
Prayer does not have to be an effort to change the laws of the universe through special pleading. The most profound prayer can be more like listening than speaking. For some, prayer is putting their personal meditation into words so it can be shared in community.
Even a complete materialist may say that we emerge from something more basic than ourselves. Remembering that we are expressions of something more basic is vital for keeping our personal lives in perspective. If waves could think, it would be vital for them to remember the ocean to keep their little circle of life in context. They would need to constantly remind themselves that they were not separate beings, but aspects of one another.
Just as some Buddhists meditate to get “Buddha Mind,” many Christians pray “in the name of Christ”, seeking the same spirit of compassion for all beings that Jesus had. Praying is a way of remembering that we are not orphans of the universe, but expressions of the mystery out of which everything comes. “Prayer” is any way we remember our larger connections to the universe and thus lift ourselves above our own isolated petty melodramas.
To pray “in Christ” means to pray until our shallow concerns deepen and broaden into a compassion for all beings. That does not mean Christianity is the only way to such compassion. It just means when we rise from prayer with the same universal compassion Jesus had, we know our prayers have been answered.