What was God's purpose in showing longsuffering to Paul first?
God's purpose was to make Paul a pattern, not merely to save him. By extending mercy and grace to Paul first, God displayed His longsuffering so that all who would afterward believe on Jesus Christ could see the pattern and by believing have life everlasting.
Les explains that Paul was the chief of sinners—not the worst, but at the head of the line. Since Paul was to play the role of the Apostle of the Gentiles, God extended mercy and grace to him first, that in him first He might show forth longsuffering for a pattern. That pattern was for those who should hereafter, from that point on, believe on Jesus Christ and by believing have life everlasting. So Paul's case became the model of God's grace and longsuffering for future believers.