In the book it says, Believers of different religions should try to identify those issues on which a shared, believing witness is possible, together with a genuine search for unity, in humble submission to Gods will.(Chapter: Religious pluralism, Christian responses, at the end of the first paragraph)
I would like to ask the author: How can people who have entirely differently or even CONTRARY teachings regarding God or Gods, as the case may be, form one unity? And ignoring that for a moment: How can they obey a teaching that lacks unity, that is, that is contradictory within itself. Where does true UNITY exist?
Answer: According to Catholic teaching: "One is the community of all peoples, one their origin, for God made the whole human race to live over the face of the earth. One also is their final goal, God, His providence, His manifestations of goodness, His saving design extend to all men (read Wis 8:1; Acts 14:7; Rom 2:6-7; 1 Tim 2:4), until that time when the elect will be united in the Holy City, the city ablaze with the glory of God, where the nations will walk in His light." (read: Acts 21:23f.)" (Declaration on the relation of the Church to non-Christian religions: Nostra Aetate of the Second Vatican Council, nr. 1).
Regarding belief in God, the same Council says in the same Declaration, Nr. 3: "The Church regards with esteem also the Moslems. They adore the one God, living and subsisting in Himself; merciful and all-powerful, the Creator of heaven and earth, who has spoken to men.
However, by no means does this mean that there are not substantial differences between Islam and Christianity teaching about God. The God of Christian belief is the God of the Bible and the God that revealed Jesus. It is the triune God of Church teaching.
Nevertheless, Christians and Muslims are united regarding the one God and the will to fulfil Gods will. We Christians have not been given the competence nor the task to say with certainty if and when a person has consciously really encountered the God of Christian revelation and has nevertheless freely and thus culpably rejected him. God alone knows each human heart.
The Catholic Church teaches that God wants everyone to receive salvation, and that a person only then loses this offer of salvation when he freely and consciously rejects the offer of Gods love in Christ. Jesus Christ is in fact the only way to salvation. However, this salvation is also realised outside the institution of the Church and baptism. Those who are just and seek God will attain eternal salvation, even if they do not know it themselves, through Christs act of salvation (meditate Mt 25:31 onwards: The Last Judgement. The just are in the poor, the imprisoned etc who encounter Christ without recognising him. See also the text of the Councils constitution regarding the Church (Lumen Gentium) nr. 16.
"Those also can attain to salvation who through no fault of their own do not know the Gospel of Christ or His Church, yet sincerely seek God and moved by grace strive by their deeds to do His will as it is known to them through the dictates of conscience."
Read once more in the book, Chapter 11, III, 4-6 (=introduction to theme 11: Religious Pluralism).