ROME — As the cardinals walk into the Sistine Chapel on Wednesday evening to open the papal conclave, they’ll chant the plaintive prayer that their forebears have sung for centuries: Veni Creator Spiritus. Come Holy Spirit.

The process of selecting a pope is fully a blend of the political and the spiritual, theologians and cardinals say, an effort to take all the humanness and experiences of the men in the conclave and partner with the Holy Spirit — in Christian belief, an equal person of the Trinity, with the Father and the Son — to decide together the leader the Church needs.