In his preface to a new book authored by Cardinal Mueller, Pope Francis stresses that money is itself a good tool when it broadens our opportunities, but it can also turn against the human person.

"Money is an instrument that in some ways … prolongs and increases the capacity of human freedom, allowing it to work in the world, to act, to bear fruit," Pope Francis wrote in the preface to "Poor for the Poor: The Mission of the Church," published in Italian by the Vatican Publishing House, and authored by Cardinal Gerhard Mueller, prefect of the Congregation for the Doctrine of the Faith.

In the book, Cardinal Mueller explains that "true liberation theology is opposed to Marxism as well as to the current economic liberalism," and he reflects on the "conversion of life" that can lead human beings to recognize each other as brothers.

The book also recalls the personal friendship between the prefect and Fr. Gustavo Gutierrez, the 'father' of the liberation theology, and includes writings by both Fr. Gutierrez and Joseph Sayer, who 25 years ago introduced Fr. Gutierrez and Cardinal Mueller.

In the preface, Pope Francis discussed the different forms of poverty, noting that "money and economic power" can "alienate man from man, confining him in an egocentric and egoistic horizon."

"When economic power is an instrument that produces treasures kept for oneself, hidden from the other, it produces iniquity, and loses its original positive value," he stressed.

He underscored his point by pointing to the Aramaic word "mammon," used by Christ for a hidden treasure, and then referring to St. Paul's use of "arpagmos." It is a robbery when "goods are used by men who apply solidarity only for their inner circle or to receive solidarity," but who never offer solidarity to others.

This is what happens when "man, having lost hope in a transcendent horizon, has lost a taste for gratuitousness, a taste for the good for its own sake."

When man is reminded of his "fundamental solidary" with all mankind, the Pope wrote, "then he knows he cannot keep for himself the goods at his disposal."

"When man lives habitually in solidarity, he is well aware that whatever he refuses to deliver, and keeps for himself, sooner or later will backfire on him."

Noting a "unique bond between profit and solidarity," the Pope said it is the task of Christians to "re-discover, live and announce to everyone the original and precious unity between profit and solidarity."

He then went on to discuss other forms of poverty: first of all, our limited nature which is always in need of help from someone else – "we did not make ourselves and we cannot give ourselves everything we need."

"The recognition of this truth invites us to remain humble and to practice with courage solidarity, with a strength indispensable to life itself."

This experience of our poverty of limitation, he wrote, leads to a conversion toward the other and a "social practice in which the common good does not remain an empty and abstract word."

"When man is understood this way and taught to live this way, his original, creaturely poverty is not felt anymore as handicap, but rather as a resource through which what enriches each person, and is freely given, is a good and a gift from which everyone can benefit."

This, the Pope wrote, is "the positive light by which the Gospel invites us to see poverty," being the basis of the beatitude of "the poor in spirit."