But they have a basic (?) understanding of who Christ is, what He has done, that they are sinners, and they believe/have faith/trust in Him. That may not be all their is, but isn't that enough?
No. However, I think you may be focusing in on what's at the heart of the issue.
Many churches that favor alter [sic] calls as the quickie way to membership in a Christian congregation really don't appear to care about much other than enrolling the person.
What he understands about the faith is essentially unimportant to that church so long as he's 'signed on' verbally. If that process is what is relied upon in order to keep the membership rolls up, I don't think it matters very much what the body count happens to be.
In addition, I have to challenge the claim that he actually does
know "who Christ is, what he has done," and etc., thanks to his participation in the routine we've been discussing. In fact, it raises another question...why would that person be expected to show up thereafter to listen to any sermons from that pastor or another one, having already passed muster on what you've suggested is all that needs to be known by him?? And why does that Bible include all those letters addressed by Paul or John, etc. to the faithful of various congregations concerning the misperceptions about true Christian belief or practice among people who were already enrolled as members?