inundado por um mistério de luz que é Deus e N´Ele vi e ouvi -A ponta da lança como chama que se desprende, toca o eixo da terra, – Ela estremece: montanhas, cidades, vilas e aldeias com os seus moradores são sepultados. - O mar, os rios e as nuvens saem dos seus limites, transbordam, inundam e arrastam consigo num redemoinho, moradias e gente em número que não se pode contar , é a purificação do mundo pelo pecado em que se mergulha. - O ódio, a ambição provocam a guerra destruidora! - Depois senti no palpitar acelerado do coração e no meu espírito o eco duma voz suave que dizia: – No tempo, uma só Fé, um só Batismo, uma só Igreja, Santa, Católica, Apostólica: - Na eternidade, o Céu! 
The letter was written in German by Card. Ratzinger and O’Leary provided a translation. However, Gerald of the Cafeteria also did a translation. After a rapid check, I will give you O’Leary’s, because the English is smoother.
NB: O’Leary calls this letter "frightening", which gives you an idea of his take on the Joseph Ratzinger and the use of the older form of Mass.
Here is the O’Leary translation with my emphases and comments:
To Dr. Heinz-Lothar Barth, 23 June 2003
Dear Dr. Barth,
I thank you cordially for your letter of April 6 to which I find the time to answer only now. You are asking me to act for a broader availability of the old Roman rite. Actually, you know yourself that I have no deaf ears towards such a request. My work on behalf of this cause is meanwhile generally known.
Whether the Holy See will “admit the old rite again for every place and without restrictions” as you desire and have heard it rumoured cannot be simply answered or confirmed without further ado. [We know more about this now, of course.] Still too great is the aversion of many Catholics, instilled in them over many years, against the traditional liturgy which they scornfully call “preconciliar”. Also one would have to reckon with considerable resistance on the part of many bishops against a general readmission. [This is pretty dense and needs to be pulled apart. 1) Immediately Card. Ratzinger wants to dispel the fiction that the use of the older Mass is somehow out of step with the Second Vatican Council. He knows that "pre-conciliar" is code for "opposed to the Council". Ratzinger sees continuity between the older Mass and Council, not rupture. 2) The aversion which was instilled in people was purposely instilled: people in power positions tried to make others hate the older form of Mass. They weren't content to make people love the newer form, they wanted people to feel aversion to the older. 3) Bishops, who should know better, are the real obstacles.]
Things look different, however, if one thinks about a limited readmission. The demand for the old liturgy is limited, too. [Ratzinger is talking about a measured response to the demand for the older Mass. Also, he is a shrewed strategist: he knows that to over-reach would do great harm to future possibilties. This is the "brick by brick" element of his Marshall Plan I keep talking about.] I know that its worth, of course, does not depend upon the demand for it, but the question of the number of interested priests and laypeople, nevertheless, plays a certain role. Besides, such a measure can now, only some 30 years after the liturgy reform of Paul VI, be implemented only stepwise. Any new hurry would surely not be a good thing. [See? It would be worse to tye to implement something that has little chance of success.]
I believe, though, that in the long term the Roman Church must have again a single Roman rite. The existence of two official rites is for bishops and priests difficult to “manage” in practice. [This is why Summorum Pontificum is so clever! Papa Ratzinger does not resolve on a scholarly lever the debate about whether or not there are two rites. Frankly, I doubt serious if Papa Ratzinger thinks that the Novus Ordo and the older form are really the same Roman Rite. What we got in Summorum Pontificum was a juridical solution to the issue. By saying there is, juridically, one Roman Rite, he eliminated the need for a priest to have additional faculties to use the older form. That was a masterstroke.] The Roman rite of the future should be a single rite, celebrated in Latin or in the vernacular, but standing completely in the tradition of the rite that has been handed down. It could take up some new elements which have proven themselves, like new feasts, some new prefaces in the Mass, an expanded lectionary – more choice than earlier, but not too much, – an “oratio fidelium”, i.e., a fixed litany of intercessions following the Oremus before the offertory where it had its place earlier. [This is huge and it needs more explanation below.]
Dear Dr. Barth, if you commit yourself to work for the cause of the liturgy in this way, you will surely not stand alone, and you will prepare "public opinion in the Church" for eventual measures in favor of an expanded use of the earlier liturgical books. One should be cautious, however, about awakening too high or maximum expectations among the traditional faithful. [See my comments above.]
I am using the opportunity to thank you for your appreciated commitment to the liturgy of the Roman Church in your books and lectures, even if here and there I would wish still more charity and understanding towards the magisterium of the pope and bishops. [Many of our liturgical problems remain battles over ecclesiology!] May the seed you are sowing germinate and bring much fruit for the renewed life of the Church the “source and summit” of which, indeed its true heart, is and must remain the liturgy.
With delight I give you the blessing you have asked and remain sincerely yours
Joseph Cardinal Ratzinger