How can a human being on earth determine who is truly in the Church?
I wasn't aware that Christ gave us a way to truly know.
Maybe there was a meeting I am unaware of??
The great St. Augustine didn't know. He commented a lot on the "visible" and "invisible" church.
(from Pope Pius XII, Mystici Corporis Christi:)
(1) A rationalistic or purely sociological understanding of the Church, according to which she is merely a human organization with structures and activities, is mistaken. The visible Church and its structures do exist but the Church is more, as she is guided by the Holy Spirit:
Although the juridical principles, on which the Church rests and is established, derive from the divine constitution given to it by Christ and contribute to the attaining of its supernatural end, nevertheless that which lifts the Society of Christians far above the whole natural order is the Spirit of our Redeemer who penetrates and fills every part of the Church.
[however,]
(2) An exclusively mystical understanding of the Church is mistaken as well, because a mystical "Christ in us" union would deify its members and mean that the acts of Christians are simultaneously the acts of Christ. The theological concept una mystica persona (one mystical person) refers not to an individual relation but to the unity of Christ with the Church and the unity of its members with him in her.
The Church subsists in the Roman Catholic Church.