A book by Pope Francis to be published tomorrow reveals the depths of his commitment to God's mercy and his rejection of legalism.

Taking the form of an interview with Italian journalist Andrea Tornielli, The Name of God is Mercy includes vivid stories of the Pope's own experience of mercy, and condemnations of corruption and pride in the Church.

The book is published just after the beginning of the Roman Catholic Church's Jubilee Year of Mercy, which Francis inaugurated in order to help bring his message to the forefront of his papal programme.

He has been criticised by conservatives for not being clear enough about the Church's teaching on issues like homosexuality and divorce. However, in the book he compares such criticisms to "angry mutterings" that Jesus also heard "from those who are only ever used to having things fit into their preconceived notions and ritual purity instead of letting themselves be surprised by reality, by a greater love or a higher standard".

He says there are two ways of thinking about faith in the Gospels. "On the one hand, there is the fear of losing the just and saved, the sheep that are already safely inside the pen," says Francis. "On the other hand, there is the desire to save the sinners, the lost, those on the other side of the fence."

"The first is the logic of the scholars of the law," he continues."The second is the logic of God, who welcomes, embraces, and transfigures evil into good, transforming and redeeming my sin, transmuting condemnation into salvation."

He refers frequently to parables of Jesus, saying once, "We must go back to the Gospel."

Taking on his conservative critics, he says: "I will say that there is often a kind of hypocrisy in them, a formal adherence to the law that hides very deep wounds." He continues: "Jesus uses tough words; he defines them as 'whited sepulchers' who appear devout from the outside, but inside, on the inside... hypocrites."

He describes them as "men who live attached to the letter of the law but who neglect love; men who only know how to close doors and draw boundaries".

He speaks passionately of the need for mercy in today's world.

"The fragility of our era is this, too: we don't believe that there is a chance for redemption; for a hand to raise you up; for an embrace to save you, forgive you, pick you up, flood you with infinite, patient, indulgent love; to put you back on your feet."

He also says he has a "special relationship" with prisoners. "I have always been very attached to them, precisely because of my awareness of being a sinner. Every time I go through the gates into a prison to celebrate Mass or for a visit, I always think: why them and not me? I should be here. I deserve to be here. Their fall could have been mine. I do not feel superior to the people who stand before me.

"And so I repeat and pray: why him and not me? It might seem shocking, but I derive consolation from Peter: he betrayed Jesus, and even so he was chosen."

In response to questions about whether there can be too much mercy, he says: "The Church condemns sin because it has to relay the truth: 'This is a sin.' But at the same time, it embraces the sinner who recognises himself as such, it welcomes him, it speaks to him of the infinite mercy of God. Jesus forgave even those who crucified and scorned him."

Quizzed on his controversial statement about gay people at a press conference in 2013, he said: "On that occasion I said this: If a person is gay and seeks out the Lord and is willing, who am I to judge that person?"

He explains: "I was paraphrasing by heart the Catechism of the Catholic Church where it says that these people should be treated with delicacy and not be marginalised."

He says that people "should not be defined only by their sexual tendencies": "Let us not forget that God loves all his creatures and we are destined to receive his infinite love."

He says: "I prefer that homosexuals come to confession, that they stay close to the Lord, and that we pray all together. You can advise them to pray, show goodwill, show them the way, and accompany them along it."