Unity was soon lost as groups left or were ejected from the original Church. The Christian Church was no longer found in one organization.
From the beginning, the apostles in union with Peter, then the bishops in union with the pope were led by the Holy Spirit to teach truth (true doctrine). This was the promise of Jesus.
The ‘church’ has to visibly exist somewhere because Jesus said it is like a city on a mountaintop which can’t be hidden. It can’t only be invisible.
It was our own sinfulness and the work of the evil one that caused the unity to be broken. When two preachers teach opposite “truths” any logical person knows that one of them is in error.
Only in one sense is there an invisible church. All those who are baptized into Christ receive the Holy Spirit and are sons of the Father and the Spirit works in all our lives. But, since different churches focus only on certain truths or teach opposing ‘truths’, some Christians believe some errors (but not knowingly) or they don’t have all the truth they could have. This is different from saying all groups contain part of the true universal Christian Church.
As a Catholic, I absolutely believe that when I receive communion I receive Jesus himself because the Catholic church has passed this power from bishop to bishop from the time of the apostles. And I believe the teachings and the bible have been passed down the same way. So the completeness of Christianity exists in the Catholic church.
Although Catholics, like others, are sinful, God has been faithful to His promise to lead His church into all truth. It isn’t that we believe ourselves to be incapable of error; instead we trust in Jesus’ promise.