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.

The Holy Spirit is like the backstage star of the conclave.

One theologian compares the concept to “the deep music in a song, the deep bass line” under the rhythm and melody.

Some cardinals use Holy Spirit-accessing rituals that sound a bit like cognitive behavioral therapy. Pope Benedict XVI said it’s more like a “good educator” who doesn’t pick the pope but encourages.

It could play a unique role this year, cardinals and theologians say. The conclave to choose a successor to Pope Francis — 133 cardinal electors — will be the largest ever. For the great majority of cardinals, it will be their first.

Those who were created in recent years — Francis named his last group in December — are unlikely to know many of their peers. Two dozen come from countries that have never participated in a conclave.

With no interpreters allowed and no group meetings once the doors are shut, cardinals say they could be relying on the Holy Spirit in a new way.

“It is the Holy Spirit who has to guide us and help us, because it’s not immediately easy to see for whom to vote,” said Cardinal Anders Arborelius, the bishop of Stockholm.

Christians differ in their understanding of the Holy Spirit

When the cardinals sing “Come, Holy Spirit,” the Rev. Louis John Cameli said, they’re really praying “for their own openness to the inspiration that the spirit might bring to them.”

People wrongly think the process is akin to “figuring things out and then coming to a decision,” said Cameli, a Chicago priest who studies spirituality, but it’s more nuanced. Humans bring intelligence and logic, he said, and the Holy Spirit helps to bring that along.

Many Catholics and non-Catholic Christians have different ways of understanding or describing it. Pentecostal Christians associate the presence of the Holy Spirit with a discernible sign, such as speaking in tongues.

The writer Anne Lamott, whose fiction and nonfiction books often explore faith, says the Holy Spirit comes to her aid when she’s “feeling tiny and vulnerable and scared.”

“I have the theological understanding of a bright second-grader,” she wrote in an email to The Washington Post, “but my understanding of the Holy Spirit is that it’s the comforter. The motherly tender feeling of Jesus’ love around me and indwelling me. Sometimes a very Jewish mother — ‘HAVE YOU EATEN?’ — and sometimes as … a very gentle cooling breath when I am tweaking and overheating with bad thoughts.”

“I call on Jesus when I am in a hole too deep to get myself out of, and the Holy Spirit wafts in, like Casper the Friendly Ghost meets Bette Midler.”

The study of the Holy Spirit is called pneumatology, which has the same Greek root as pneumonia: breath.

People asked about the subject can bristle — as some did for this article — because they prefer not to think of it as something explainable. Especially when talking about the forces that pick a new pope.

Chanting “Come, Holy Spirit,” the Catholic University canon lawyer Kurt Martens said, “emphasizes they are not just casting ballots. The ballots are the results of prayer and reflection, but not in a Harry Potter way.”

‘God is deadly serious about human freedom’

Catholic theologians emphasized that the cardinals participate fully in the selection of the pope; they’re partners with the Holy Spirit. The concept applies also to the people who wrote the Bible.

The human authors of scripture “aren’t scribes. They aren’t court reporters,” said the Rev. Peter Folan, a theologian at Georgetown University. “God is deadly serious about human freedom.”

If the cardinals and their lives and relationships with one another weren’t essential, Folan asked, why bother having a conclave?

Popes the Holy Spirit ‘obviously would not have picked’

But can the Holy Spirit make a mistake? How do you know you’re hearing the Holy Spirit and not an evil spirit?

The spirit is God, and it is reliable, said Cameli. “We’re not so reliable. We can misinterpret, be inattentive, be so absorbed in our own world that we throw blocks and resistance.

The problem is not on the side of God, who wants the best for us.”

Benedict, when he was Cardinal Joseph Ratzinger, told Bavarian television in 1997 that he wouldn’t say “the Holy Spirit picks out the pope. … I would say that the spirit does not exactly take control of the affair, but rather like a good educator, as it were, leaves us much space, much freedom, without entirely abandoning us.”

“Probably the only assurance he offers is that the thing cannot be totally ruined,” he said. “There are too many contrary instances of popes the Holy Spirit obviously would not have picked!”

Trying to access the spirit

The cardinals in the conclave will use many spiritual tools and prayer practices to try to access the Holy Spirit.

One, attributed to Saint Ignatius of Loyola, the founder of the Society of Jesus — the Jesuits — is the Examen, an exercise that has attributes of cognitive behavioral therapy in that practitioners ask themselves about their behaviors, feelings and thoughts in a systematic way.

The Examen, Jesuits say, calls for people “to place themselves in God’s presence … and pray for the grace to understand how God is acting in your life.” Then they review their day, recall moments and feelings at the time, and reflect on a question: “Were you drawing closer to God or further away?”

Most of the time ‘God doesn’t answer audibly’

The Holy Spirit “is symbolized by breath, wind and fire, [and] is not an impersonal force,” Cardinal Michael F. Czerny, a Canadian Jesuit, wrote to The Post. “It’s both a very human and a very spiritual experience.”

“The Holy Spirit is at work in the congregations and the conclave,” he wrote. “The cardinals are praying, celebrating Mass, invoking the Holy Spirit, and, very important, too: listening to one another, reflecting personally and dialoguing together.”

Cardinal Timothy Dolan of New York, writing in 2013 on the conclave that elected Francis, praised the prayers of Saint Ignatius. “If you put a lot of thought, time, and prayer into making a decision, once you make it — tough as it is — you’ll sense some interior serenity and peace.”

But “most of the time,” he wrote, “God doesn’t answer audibly. When people say they actually hear God’s voice, we usually call Bellevue Hospital and ask if they’ve got room for another patient.”

A belief that has evolved over time

The concept of the Holy Spirit has evolved from the days of the early church. In the first centuries, early Christians were still trying to establish and understand what the Trinity meant.

“There was development of what people thought Jesus was,” Folan said. “Is this a second god? So are we not monotheists anymore? … Then what do you do with the spirit? All of these things develop.”

The concept of “spirit” has become much wider and blurrier in modern life. “Spiritual but not religious,” according to sociologists, is one of the biggest segments of the American belief landscape.

The spiritual writer Frederick Buechner wrote in his 1973 book, “Wishful Thinking,” that the word spirit “has come to mean something pale and shapeless, like an unmade bed. School spirit, the American spirit, the Christmas spirit, the Spirit of ‘76, the Holy Spirit — each of these points to something that you know is supposed to get you to your feet cheering, but that you somehow can’t rise to. The adjective spiritual has become downright offensive.”

In the Sistine Chapel on Wednesday, the cardinals will call to the Holy Spirit with different images. “Living fountain, fire, love … finger of the Father’s right hand,” the prayer reads in Latin. “Inflame our senses with Thy light, pour Thy love into our hearts, strengthen our weak bodies with lasting power.”

Sally Jenkins contributed to this report.