By attending a motu proprio Mass, a person implicitly shows that he accepts the new mass because the motu proprio Mass is allowed only under this condition.
Former Pope Benedict XVI showed this condition when he wrote in his July 7, 2007 letter: “Needless to say, in order to experience full communion, also the priests of the communities adhering to the former usage cannot, as a matter of principle, exclude celebrating according to the new books. The total exclusion of the new rite would not in fact be consistent with the recognition of its value and holiness.” And again: “[T]he new missal will certainly remain the ordinary form of the Roman rite . . . on account of the juridical norms”.
Further, former Pope Benedict XVI wrote: “The faithful who ask for the celebration of the forma extraordinaria must not in any way support or belong to groups which show themselves to be against the validity or legitimacy of the Holy Mass or the Sacraments celebrated in the forma ordinaria .” May 13, 2011 Instruction “Universae Ecclesiae” on the implementation of the Motu Proprio “Summorum Pontificum”, ¶19.
Cardinal Camille Perl (Secretary of the Ecclesia Dei Commission for Indult groups) candidly disclosed:
In the original document granting permission for the traditional Latin Mass, Quattuor abhinc annos of 3 October 1984, the Church made a condition for the concession of the use of the 1962 Roman Missal that one must not “call into question the legitimacy and doctrinal exactitude of the Roman Missal promulgated by Pope Paul VI in 1970.” While the other conditions required by that document have been modified in the light of the Apostolic Letter Ecclesia Dei of 2 July 1988, that first condition retains its full effect. Those who are unwilling to recognize the legitimacy and doctrinal exactitude of the present Roman Missal are not entitled to the celebration of the so-called "Tridentine" Mass. This is a very serious and non-negotiable matter.
Quoted in: http://archives.sspx.org/motu_proprio/legitimacy_of_quo_primum_today.htm
Here is a hypothetical example to illustrate the danger of implying acceptance of evil: Suppose a rich man announced that he would distribute free food in the town plaza, but he invited the participation of only those persons who acknowledge that the devil is God. By a person’s presence, while silently participating in this food giveaway, he implicitly indicates his agreement that the devil is God.
The motu proprio Mass is offered by priests who commit the sacrilege of offering the new mass. See, e.g., former Pope Benedict XVI's July 7, 2007 apostolic letter, Summorum Pontificum, Art. 2, requiring use of the new mass on some days.
Would you attend the motu proprio Mass offered by a priest who also offers a Black Mass? If not, then why accept a priest who ever offers the sacrilegious new mass? The new mass is inherently harmful to souls and to the Catholic Faith. Thus, it is a “sacrilege” because it fits the definition of “sacrilege”: viz., the “irreverent treatment of the sacred”. Summa, IIa IIae, Q.99, a.1.
The motu proprio Mass is offered only by priests who refuse to condemn the worst religious errors infecting the Church in our times. Just as the Catholics of the Fourth Century were right to follow St. Athanasius and avoid priests who refused to condemn Arianism (even though those priests offered the very same Mass), then we should not attend motu proprio Masses, which are offered only by priests who refuse to publicly condemn the principal religious errors of our times!
If a person attends a motu proprio Mass in a church that is also used for the new mass, then he willingly uses for worship, a place which is also set aside for the commission of sacrilege. A place used for sacrilege is far more unfitting for Divine worship than a “neutral” place, like a rented meeting hall.
By attending a Mass set up under the pope’s 7-7-07 motu proprio, a person implicitly affirms the falsehood that the pope’s motu proprio (or the 1984 or 1988 indults) is/was needed for the traditional Mass.
When the traditional Mass and the new mass are both offered in one church, people entering that church for the traditional Mass will give the impression that they also attend the new mass, because it is reasonably foreseeable that they will be seen entering the church by persons who will not take note of the time of entry. This foreseeable scandal is similar to that caused by persons attending an innocent bingo game in a location shared with a business offering pornographic entertainment, even if the bingo game occurs earlier in the morning than when that porn entertainment is not offered.
For hosts not consecrated at that particular motu proprio Mass, it might be unknown if they are valid and whether their consecration occurred in a sacrilegious new mass.
The “priests” offering the motu proprio Mass usually were “ordained” using the conciliar rite of “ordination” or were “ordained” by a “bishop” who was “consecrated” by the conciliar rite of consecration. Such “ordinations” are doubtful.
By attending a motu proprio Mass (the so-called “extraordinary form”), a person implicitly shows that he accepts the new mass (the “ordinary” form) as good. The new mass cannot be the “ordinary” option (or even an option at all), unless it is good.
Attending the motu proprio Mass cuts you off from the uncompromising doctrine and sermons of heroic traditional priests who offer the non-motu proprio Masses.
Attending the motu proprio Mass causes the gradual tendency to go to the motu proprio priests for the new rite of confession and other new sacraments (each of which has conciliar “taint” and other serious problems).
Attending the motu proprio Mass makes other conciliar compromises easier and more likely, e.g., recitation of the “Luminous” mysteries of the rosary or the participation in the conciliar “King of Mercy” devotion.
Attending the motu proprio Mass makes you comfortable with other aspects of the conciliar milieu, e.g., conciliar church architecture, with a detached tabernacle and a Protestant/conciliar table instead of an altar, to emphasize the mass as a meal (as Luther and Cranmer did).
Attending the motu proprio Mass causes a gradual tendency to view the motu proprio priests as a source of good advice. Thus, you will get advice from priests who consider the new mass and Vatican II to be good and who consider uncompromising traditional Catholics to be in schism.
Donating money at a motu proprio Mass financially promotes harmful conciliar causes, at least indirectly. Even when your donations are stated to be for a particular “unobjectionable” cause (such as paying for a new roof on the church), such donations still indirectly promote harmful causes because your donations free up other money to be used to promote those harmful causes. (This indirect effect is similar to donating money to a Protestant organization which both teaches classes in Protestantism and also teaches cooking classes. If you donate money with the stipulation that your money be used only for cooking classes, your donation merely frees up other money to be used to teach Protestantism.)
When one attends the motu proprio Mass, he fails to give his moral and financial support to non-motu proprio priests who are courageously fighting for Catholic tradition. You advance the conciliar goal of siphoning-off support from those uncompromising priests.
Priests offering the motu proprio Mass are often so far from a Catholic mindset that they often prefer the new mass and offer the motu proprio Mass only because they are assigned to do so.
Attending the motu proprio Mass will affect who you associate with (and, if you are looking for a spouse, will affect who you marry). All of us need the help of strong Catholic friends, to aid us in living the traditional Catholic life more fully. If you attend the motu proprio Mass, your friends (and future spouse) will have the motu proprio mindset, of accepting the new mass as good, accepting the conciliar teachings as true, and rejecting the uncompromising (non-motu proprio) traditional Mass, traditional Faith and traditional Sacraments. The influences upon you will be motu proprio friends at a conciliar parish.
Attending the motu proprio Mass is to knowingly walking into a trap set by (and candidly disclosed by) the conciliar hierarchy. The pope and cardinals have clearly said that they continue on the course of post-conciliar errors and that these motu proprio Masses are designed to integrate traditional Catholics into the conciliar church. See, e.g., http://www.zenit.org/article-32593?l=english (Vatican calling the traditional Mass an “ecumenical bridge” and outlining the plan for using it as a step to blending the new mass and the traditional Mass); & http://www.catholicnews.com/data/stories/cns/1101922.htm.
As Pope John Paul II admitted: “The Holy See has granted... the faculty of using the liturgical books in use in 1962... It is very evident that, far from seeking to put a brake on the application of the reform undertaken after the Council, this concession is destined to facilitate the ecclesial communion of people who feel themselves attached to these liturgical forms.” Audience of Sept 28, 1990 to the Benedictine Monks of Le Barroux. L'Osservatore Romano (French edition), Oct 2, 1990, No. 40 (emphasis added).
Cardinal Mayer (former president of the Ecclesia Dei Commission in charge of indult groups) candidly admitted the conciliar trap: “There are grounds to hope that, with the concerted efforts on the part of all concerned, a substantial number of priests and seminarians will find the strength to renounce a 'state of mind' which until now was full of prejudices, of accusations and of disinformation .... We have good reason to believe that the charity with which the priests coming from Archbishop Lefebvre and returning into the Church will be received, will contribute greatly to the fulfillment of this hope that, following them, numerous faithful whom they had served up till then, would also return into the ecclesial communion [with the conciliar church] through their mediation. Sometimes a temporary solution may be necessary, such as allowing them the possibility of celebrating the Holy Mass [of Pope St. Pius V].” See, Letter of Cardinal Mayer to Msgr. May, L'Homme Nouveau, March 19, 1989 (emphasis added), quoted at: http://www.sspx.org/motu_proprio/attendance_at_the_indult_vanes.htm.
Thus, when traditional Catholics choose a motu proprio Mass, they knowingly accept the conciliar hierarchy’s bait and they presume they will be strong enough and wise enough to discern and reject the accompanying conciliar poison. This is like a person knowingly eating pieces of a poisoned apple pie because he presumes that he will never fail to recognize (and avoid) the poison. By attending the motu proprio Mass, one is wagering his eternal salvation that he can outsmart the conciliar hierarchy.
Back when Fr. Peter Scott fearlessly defended Catholic Tradition, he wrote: “The motu proprio Mass, like the indult Mass, is therefore not for traditional Catholics.” See, Fr. Peter Scott Q & A “Can the faithful assist at the traditional Masses celebrated in virtue of the motu proprio of Pope Benedict XVI of July 7, 2007?” http://op54rosary.ning.com/forum/topics/can-the-faithful-assist-at-the-traditional-masses-celebrated-in-v?xg_source=activity 6