Here we go with Protestant false dichotomy, the Christian religion is zero sum and one dimensional, if we say there is security in the flock, somehow, for some reason, this is an affront to Christ and his "all sufficient work" on the cross, blah, blah, blah.
Our security is our Savior.
The Church is the Body of Christ. Thus, to be in the "flock" is to be part of the body, and thus united with Christ. Because the Church is the Body of Christ, there is a connection between Christ's presence on earth and the Church. The Body of Christ mediates the presence of Christ in the world. Thus, the Church is a very necessary component in one's relationship with Christ.
You have it backwards.
One is not in Christ because he joined a flock.
One is part of his flock because he is in Christ.