Why did Paul only refer back to the resurrected Christ and not his earthly ministry?
Paul's sole message was the death, burial, and resurrection of Christ—the preaching of the cross, not His earthly miracles. Christ's earthly ministry spoke to Israel under the law, but Paul reveals the same Jesus from His glorified, ascended, resurrected position, where God's power is made evident.
Les explains that Paul never mentions Bethlehem, Christ's birth, earthly ministry, or miracles. Paul has only one message: the death, burial, and resurrection of Christ, the shed blood, which is the core of all his epistles. He alludes to the preaching of the cross, not the preaching of Christ and His miracles. Les relates a conversation with a lady who said we must go by what Jesus said; he cautioned that what Jesus said in His earthly ministry was said to the Jew under the law. When we go to Paul's epistles, Paul reveals the very words of the same Jesus from His glorified, ascended position as the resurrected Lord and Savior. That makes a big difference. Therefore Paul never refers back to his earthly ministry, but only to the resurrected Christ, where the power of God was made evident. The preaching of the cross is foolishness to those who perish, but to those saved it is the power of God, just as God opened the Red Sea and raised Christ and brings us out of deadness in sin.