Why do believers no longer focus on Christ's earthly ministry?
Believers no longer center on Christ's earthly ministry because their salvation rests entirely on His death, burial, and resurrection. That finished work, applied by the Holy Spirit, makes every believer a new creation and baptizes them into one body, so faith hangs on the gospel accomplished for them rather than on His earthly ministry.
Les explains that since we are no longer hanging on Christ's earthly ministry, we are hanging everything on His death, burial, and resurrection. Therefore, even though all the sins of the world have been paid for, salvation is conditional: if anyone be in Christ, he is a new creation. Les notes that creation and creature are the same word in the Greek, but creation fits English better because the believer is not just made a new creature but is a whole new creation of God. That is what makes a believer a believer, and unless one has experienced it, one is not a believer. There must be that new creative work of the Holy Spirit in the believer's life. Then Scripture says in I Corinthians 12:13 that by one Spirit all believers, not just the best or most spiritual or those who have had a particular experience, are baptized into one body.
Wherefore henceforth know we no man after the flesh: yea, though we have known Christ after the flesh, yet now henceforth know we him no more.
More Questions
What should believers focus on instead of Christ's earthly ministry?
Believers are not to concern themselves that much with Christ's earthly ministry, but rather with the work of the cross, because that is where everything comes to a head.