CathInfo.com

the Holy Sacrifice of the Mass is the supreme expression of a doctrinal and moral universe. It is therefore a question of choosing the Catholic faith in its entirety and through it, choosing Our Lord Jesus Christ, with His Cross, His Sacrifice and His universal kingship. It is a matter of choosing His Precious Blood, of imitating the Crucified One and of following Him to the end, by a complete, rigorous and coherent fidelity.

Beautifully put, and precisely what needs to be said contra those "conservatives" of the NO who insist that the Novus Ordo Missae is just as meritorious as the true Mass.

it is indeed a war between two different and opposing conceptions of the Catholic Church and of Christian life

This letter was very wimpy, indirect and full of partial truths.  The above sentence sums it all up.  Total garbage of a letter.
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The new-sspx has failed to realize (or won't speak clearly for various reasons) that those who oppose the True Church are godless, atheistic, communists.  These evil, satanic men don't have an "opposing view of the Church" (which implies they are Christian), they are opposed to the Church existing at all.  They want to DESTROY Christianity, not simply change certain aspects of it.  They are entirely opposed to Christ, not simply wanting to "fix" broken aspects of the Church (i.e. like protestants proclaim).
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This letter speaks of the age-old battle between satan and the Church, but then waters down the implications of the battle (Trads vs Modernists) as if it's merely a battle of perspective, as if the Modernists actually have good-will, as if the Modernists want to reform the Church. 
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Pope St Pius X knew the Modernists wanted to destroy the Church, because they hate God.  He told us that "Modernists should be beaten with fists."  It's as if the new-sspx is being loaded onto a Communist Russian railcar, headed for a Siberian death camp, and they are telling everyone to calm down, because the differences with the communist party "can still be worked out".  They have yet to understand, or won't preach the truth to their faithful, of the danger these men pose to the Church and souls.   Oh, the naivety.  Oh, the stupidity.  

By the way, notice that nowhere in the letter does the word “Modernist“ appear.

I noticed that too. No mention of Vatican ll either.