Is simply understanding Jesus's earthly ministry enough to be saved?
No. Simply understanding Jesus's earthly ministry is not enough for salvation. Salvation requires going to the cross and the resurrection, and knowing Christ beyond His earthly ministry. Otherwise, as Paul says, we know Him for nothing, and we must go beyond that to know Him as the Christ after the resurrection.
Les explains that Paul says henceforth we know no man, not even Jesus, after the flesh. Paul had known Christ after the flesh—he knew who Jesus was, what He was doing, and where He came from—but he did not truly know Him. Paul can rightly say, yes, we knew Christ in the flesh, yet now we know Him no more. Les says this flies in the face of much preaching today that presents Jesus only in His earthly ministry. That is all well and good as far as it goes, but there is no salvation in simply understanding His earthly ministry. We have to go to the cross and to the resurrection; otherwise, as Paul says, we know Him for nothing, and we must go beyond that. As we know Him as the Christ after the resurrection, that web has been cut off.