This
Tu Quoque argument has been refuted quite eloquently by Jimmy Akin
here.
He says: First, Catholicism has a functioning magisterium that can decide that these matters are not essential differences. Second, the relevant schools adhere to the teachings of the magisterium and, if their views were reprobated, would accept the results (or cease to be faithful Catholics). Third, the differences between Catholic schools of thought have nowhere near the magnitude of the difference among Protestant schools. Compared to the differences among Protestant groups, differences among orthodox Catholic groups are trivial. Finally, the fact that the Catholic Church has a magisterium means that there can be—and on the most important theological matters there is—an official Catholic position. There is no parallel standard in Protestant circles that can speak for Protestantism.
Other times, advocates of the “unity in essentials” argument will attempt to construct a parallel argument against Catholics by pointing to the existence of ostensible Catholics who refuse to acknowledge the magisterium’s teachings.
In response, it should be pointed out that the difference among Catholic dissidents is frequently far less than among individuals in the sweep of Protestant belief. Furthermore, individuals’ refusal to accept the magisterium’s teachings does not challenge the Christ-given authority of the magisterium any more than the refusal to accept some of Paul’s teachings undermines Paul’s authority as an apostle. Moreover, Protestant churches also have individuals who refuse to honor the teachings of their denominations. The difference is that for Catholicism there is a body—the bishops teaching in union with the pope—who “speak for the Church” and who can articulate what “the Catholic position” is, while in Protestantism there is nothing comparable.