You realize that I'm not Catholic, right? (That's a pretty uncharitable remark, regardless.)
And you know what? People can be Christ followers in isolation. Indeed, many people are forced into it due to circumstances beyond their control.
Exceptions (that one can be a Christ-follower in isolation because of external forces) do not prove the rule ("'Holy solitaries' is a phrase no more consistent with the gospel than holy adulterers. The gospel of Christ knows no religion, but social; no holiness but social holiness," as John Wesley puts it).
A person does NOT need a church, a denomination, an organization, to follow Christ.
The Early Church Fathers disagree with you (see again Cyprian of Carthage, among others). For that matter, the Bible disagrees with you (Hebrews 10:25). Organizations naturally arise out of community, and the Church (=ekklesia, assembly of the called) is naturally a community of believers. That organization is there to regulate the shared life of the Christian community to the end that God is glorified.
A follower of Christ just needs the leading of the Holy Spirit who is the indwelling presence in the believer.
And how do you know the Holy Spirit is leading you?
A Christian is one who's one inwardly, not one outwardly based on being a card carrying member of XYZ church.
This is another false dichotomy. "Card carrying membership" may not be required, but deliberately isolating yourself from other Christians can and should call into question whether you are really a Christian inwardly.
If you are unwilling to live with your brothers and sisters in community in the context of a local church and its attendant connections to other church congregations (be that a denomination or a 'non-denominational' church's connections to other churches in the community), then you should reevaluate to what it is that Christ is calling you.
It doesn't say you have to go to church every Sunday or you're sinning. It doesn't even say to attend church. It just says assembling together and doesn't specify how often and where.
And how do you know the Holy Spirit is leading you?
What's that got to do with church attendance?
People who go to churches are not necessarily more likely to know they're being led by the Holy Spirit and people who don't go to churches are not necessarily less likely to know the same.
If you can't tell if you're being led by the Holy Spirit and how to tell, them going to church isn't going to help any.
"Card carrying membership" may not be required, but deliberately isolating yourself from other Christians can and should call into question whether you are really a Christian inwardly.
Why?
And who said that those who are not going to church are "deliberately isolating" themselves from other Christians?
You do realize, don't you, that there are other venues for getting together with other believers?
What makes going to church so special that people can't afford to miss it? How does it prove you're a Christian?
So the Mafia hit man who goes to mass every Sunday has less reason to doubt he's a Christians simply because he goes to mass?
Is Christianity really all about going to church?