In the beginning of 1 Corinthians, Paul certainly alludes to a Church that includes all those who are "sanctified in Christ Jesus, [and] called to be saints."
1 Corinthians 1:2 To the church of God that is in Corinth,
to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours:
Yet in Galatians 1:6-9 and 2:4, Ephesians 4:14, Titus 1:9-16, and other places, Paul makes it perfectly clear that not all those who gathered together were true Christians and that false believers and false teachers were also among them. Our Lord even warned of false teachers who would try to destroy the faith of the redeemed.
Matthew 24:24 For false christs and false prophets will arise and perform great signs and wonders, so as to lead astray, if possible, even the elect.
The universal body of Christ, the Church Catholic, is made up of all those who have been graciously justified by God through faith in Christ alone. Only God knows for certain who these are and whether true saving faith in Christ is present in them.
These members of the one true Church stand side-by-side with hypocrites, false teachers, deniers of Christ, wolves in sheep's clothing, and those whose faith looks not to Christ's work on their behalf alone but to their own good works for their justification. This true Church is invisible to the human eye and quite often imperceptible to the human heart.
As we confess in the Apostle's Creed, we believe in the "communion of saints" and "the holy catholic (universal) church." In the Nicene Creed we confess that we believe in "one holy catholic and apostolic church."
That is why it is known as the Invisible Church, it is invisible to the human eye and we must accept it as an article of faith.