If it is "secretive", it is not Catholic.

If there is a secretiveness, it may also be because being a traditional Catholic can feel out of place.

Question 13
What are we to think
of the Fraternity of St. Peter?
Since the introduction of the new sacramental rites, Rome had allowed no religious society or congregation exclusive use of the older rites. Then on June 30, 1988, Archbishop Lefebvre consecrated four bishops to ensure the survival of the traditional priesthood and sacraments, and especially of the traditional Latin Mass.

Suddenly, within two days, Pope John Paul II recognized (Ecclesia Dei Afflicta, July 2, 1988) the “rightful aspirations” (for these things) of those who wouldn’t support Archbishop Lefebvre’s stance, and offered to give to them what he had always refused the Archbishop. A dozen or so priests of the SSPX accepted this “good will” and broke away to found the Fraternity of St. Peter (FSSP).

The Fraternity of St. Peter is founded upon more than questionable principles, for the following reasons:

It accepts that the Conciliar Church has the power:

to take away the Mass of all time (for the Novus Ordo Missae is not another form of this, question 5),
to grant it to those only who accept the same Conciliar Church’s novel orientations (in life, belief, structures),
to declare non-Catholic those who deny this by word or deed (An interpretation of "Everyone should be aware that formal adherence to the schism [of Archbishop Lefebvre] is a grave offense against God and carries the penalty of excommunication." Ecclesia Dei Afflicata), and,
to professes itself in a certain way in communion with anyone calling himself “Christian,” and yet to declare itself out of communion with Catholics whose sole crime is wanting to remain Catholic (Vatican II, e.g., Lumen Gentium, §15; Unitatis Redintegratio §3).
In practice, the priests of the Fraternity, having recourse to a Novus Ordo bishop willing to permit the traditional rites and willing to ordain their candidates, they are forced to abandon the fight against the new religion which is being installed:

they reject the Novus Ordo Missae only because it is not their “spirituality” and claim the traditional Latin Mass only in virtue of their “charism” acknowledged them by the pope,
they seek to ingratiate themselves with the local bishops, praising them for the least sign of Catholic spirit and keeping quiet on their modernist deviations (unless perhaps it is a question of a diocese where they have no hopes of starting up), even though by doing so they end up encouraging them along their wrong path, and
note, for example, the Fraternity’s whole-hearted acceptance of the (New) Catechism of the Catholic Church (question 14), acceptance of Novus Ordo professors in their seminaries, and blanket acceptance of Vatican II’s orthodoxy (question 6).
They are therefore Conciliar Catholics and not traditional Catholics.

This being so, attending their Mass is:

accepting the compromise on which they are based,
accepting the direction taken by the Conciliar Church and the consequent destruction of the Catholic Faith and practices, and
accepting, in particular, the lawfulness and doctrinal soundness of the Novus Ordo Missae and Vatican II.
That is why a Catholic ought not to attend their Masses.

FSSP come under the authority of the Dioceses, and known as indult.

I am in understanding that the dioceses, counciliar ordinations are invalid. And all dioceses accept marxism.