1) What is Paul referring to in verse 16 when he uses the connecting word “then”?
The epistle genre of letter-writing was common in Jewish culture. All the known authors of the biblical epistles were either an apostle (meaning they were an original disciple of Jesus) or a close relative of Jesus. These men were all uniquely inspired and equipped by the Spirit of God to relay messages to the people of God through their writing. It’s probable that Paul wrote at least three letters to the Corinthian church. 1 Corinthians 5:9 refers to a previous letter; and in 2 Corinthians 2:3-4, he references information which was not covered in the first letter. Bible scholars believe there was at least one other letter, probably written between 1 Corinthians and 2 Corinthians.
It is essential in the study of 2 Corinthians 5 to consider its context within the letter as a whole, as well as his original letter to the people in Corinth (1 Corinthians). As he often does, Paul is expressing one extended thought regarding the gospel’s work in the life of a believer. In these letters, he uses the words “for” and “therefore” and “so”. Paul’s use of “then” in verse 16 refers the reader to what he said in the previous sentences regarding our relationship with Christ (verse 15). Because of His death, believers are those “who should no longer live for themselves, but for the one who died for them and was raised.” This was the essence of all of Paul’s writing. And of his living. (Romans 8:9-11, Galatians 2:8-21 Ephesians 2:4-8)

2) What does it mean for us to be a new creation once we are in Christ (verse 17)?
Most dictionaries define a Christian as “a person professing belief in Jesus as the Christ or in the religion based on His teachings.” Unfortunately, the English dictionary falls short of communicating what being a Christian really is. The word “Christian” is used only three times in the New Testament (Acts 11:21-26 ; Acts 26:25-29 ; 1 Peter 4:13-16), and was not meant as a compliment. At the time, it was common for Greeks to mockingly bestow nicknames to specific groups. Therefore, those who had followed Jesus – the one claiming to be the Christ – in their behavior, activity, and speech, were identified with Christ and were dubbed “little christs.”
In verse 17, Paul gives a definition of what it really means to be a Christian. His writing, teaching, and his own life, focused on the truth that a person who was ”in Christ” most significantly demonstrates his new identity by inwardly rejecting the (old) self-absorption into which each person is born. As a “new creation,” the believer’s mindset/heart enthrones Christ, not self. Paul’s aim was to persuade the Corinthians to deny themselves and the sinful habits attached to a self-focused life.

3) Who and what is God reconciling, and how does this shape our present reality regarding our relationships (verses 18-19)?
Scripture teaches the believer’s life is not worldly (verse 16). Once someone has become a “new creation,” he or she is in union with God and His kingdom purposes, just as Christ is. Although there will one day be an ultimate reconciling of everything to God (Acts 3:20-22 Colossians 1:19-20), God sent Jesus to become human in order to redeem the human race He created. Paul mentioned this in his first letter. (1 Corinthians 1:28-31) Every person is in desperate need of being reconciled with God because sin broke the relationship God had established with man, separating us from Him forever (Isaiah 59:2). Humanity, which was once God’s good creation, became God’s enemy because of sin. Thankfully, unlike every other religion in the world which dismisses the requirements of the moral law for mercy’s sake, Christianity is the demonstration of God’s mercy through His justice. The very sin that alienated every person from the Creator was also atoned for through Christ’s sacrificial death. (Romans 3:24-26 ) Instead of counting the trespasses of man against him, which was justified action, God Himself took the punishment (through Christ) so those who believe can be reconciled to Him. In response, the reconciled become part of God’s work in the world to reconcile others to Him.

4) What makes us “ambassadors for Christ,” and how should it affect our lives to be His representative (verse 20)?
Merriam-Webster defines an ambassador as one who is “accredited to another sovereign state … as the resident representative of their own government or sovereign.” Referring to himself as Christ’s ambassador, Paul saw himself and all believers as representatives of The Sovereign Lord. (Ephesians 6:18-20). Constantly aware of the grace of God, Paul’s letters indicate urgency as he pleads with the Corinthians to know and proclaim Christ crucified. (1 Corinthians 15:9-10, 2 Corinthians 4:14-15, 2 Corinthians 5:9-11 ) Paul continues his plea in Chapter 6, reminding the reader that God’s grace is to be received and applied to their lives, not ignored. As a new creation, a believer is intended to be a living gospel proclamation of Christ’s work on the cross on the sinner’s behalf. (Romans 12:1)
“Christ’s ambassadors come in God’s name, with His entreaties, and act in Christ’s stead, doing the very thing He did when He was upon this earth, and what He wills to be done now that He is in heaven.” (Matthew Henry’s Commentary on the Bible).