In 1917, Our Lady of Fatima said,
"In the end, My Immaculate Heart will triumph. The Holy Father will consecrate Russia to Me, and she will be converted, and a period of peace will be granted to the world."
What is the meaning of "converted"?
The greatest of all Fatima experts, Fr. Joaquin Alonso (the official archivist of Fatima), who had talked with Sister Lucy many times:
"The 'conversion' of Russia is not to be limited to the return of the Russian people to the Orthodox Christian religions, rejecting the Marxist atheism of the Soviets, but rather, it refers purely, plainly and simply to the total, integral conversion of Russia to the one true Church of Christ, the Catholic Church."
That statement seems clear enough.
And this is fully in line with the papal teachings of that period:
Pope Pius XI clearly stated:
"For if, as they continually state, they long to be united with Us and ours, why do they not hasten to enter the Church, the Mother and mistress of all Christ's faithful?" ~Pope Pius XI, in Mortalium animos 1928.
Pope Pius XII warned about confusion regarding the one true Church, the Roman Catholic Church:
"The Catholic doctrine will have to be proposed and expressed totally and integrally: what the Catholic Church teaches about the true nature and means of justification, about the constitution of the Church, about the primacy of the jurisdiction of the Roman Pontiff, about the only true union which is accomplished with the return of the dissidents to the only true Church of Christ, must not be passed over in silence or covered over in ambiguous words." ~ Pope Pius XII, in On the Ecumenical Movement 1949.
In the context of its times and certainly as it was taught through the 1950s and 60s (the times I remember) the conversion of Russia was certainly to Roman Catholicism. This, after all, was genuinely considered by Catholics of that time, as the greatest of all possible blessings.