Ladislaus, your translation of Gregory VII's letter is wrong. Gregory VII does not say that the Muslim king worships the same God (eundem Deum) as Catholics. Rather, he says that he and the king both confess one God (unum Deum). The two are quite different.
In an attempt to be friendly to a king who had helped Christians, he said that we confess one God. It is true that Muslims confess, or claim to believe, one God. It was also a non-dogmatic letter to a king who was a potential convert, not a formal statement of doctrine for the entire Church about Islam. Gregory VII taught that there is no salvation outside the Church.
Vatican II's statement is quite different and heretical. In addition to teaching that Muslims worship the same God as Catholics (which is blasphemous and false), Vatican II esteemed Islam itself (and the Muslims collectively) in view of their religious practices. That is heresy. You can actually hear a debate on this text of Vatican II, in which it is proven that Benedict XVI himself admitted that Vatican II esteemed the RELIGION of Islam itself.
In other words, the heretical nature of Vatican II's teaching on Islam in Nostra Aetate is proven by those who enforced Vatican II itself:
Benedict XVI, Address, Dec. 22, 2006: “My visit to Turkey afforded me the opportunity to show also publicly my respect FOR THE ISLAMIC RELIGION, a respect, moreover, which the Second Vatican Council (declaration Nostra Aetate #3) pointed out to us as an attitude that is only right.”
Benedict XVI, Catechesis, August 24, 2005: “This year is also the 40th anniversary of the conciliar Declaration Nostra Aetate, which has ushered in a new season of dialogue and spiritual solidarity between Jews and Christians, as well as ESTEEM for the other great religious traditions. Islam occupies a special place among them.”
Notice that Benedict XVI admitted that Nostra Aetate taught esteem for the false religion of Islam itself. Esteeming (and hence approving) a religion the Church officially considers to be abominable and diabolical is heresy.
You also apparently ignored my previous post which focused on a different heresy in Vatican II.
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There are numerous heresies in Vatican II. To name just one: the Church declares that whoever dissents from the Papacy or another dogma is ALIEN to the Body of Christ and separated from the Lord - i.e. the person is not in the Lord (Council of Florence; Leo XIII; etc.).
Vatican II teaches the opposite in the Decree on Ecuмenism: that baptized people who DISSENT from Catholic teaching, including on the Papacy or in matters of Church structure, etc. are in the Body of Christ and in the Lord. That is heresy. Vatican II even uses the word 'dissensions', indicating that it's referring specifically to people who DISSENT from Catholic teaching on the Papacy or something else. Even though such baptized people dissent from Catholic teaching on the Papacy or something else, they are, nevertheless, in the Body of Christ and in the Lord, according to Vatican II (simply because they were baptized). That is contrary to Catholic dogma. It is heresy. This video carefully examines Vatican II's text and shows that it is heretical:
Vatican II's Protestant Heresy