What do those who believe on Jesus Christ receive?
Those who believe on Jesus Christ receive life everlasting. The passage explains that from that point on, those who believe on Him—that is, on Jesus Christ—by believing have life everlasting. This is the promise given to all who believe.
Les explains that Paul, as the Apostle of the Gentiles, was shown mercy and longsuffering as a pattern for those who would hereafter believe on Jesus Christ. By believing on Him, they receive life everlasting. Les emphasizes how plain this is and connects it to Paul's defense of his apostleship and the mysteries revealed to him.