5 This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.

16 Howbeit for this cause I obtained mercy, that in me first Jesus Christ might shew forth all longsuffering, for a pattern to them which should hereafter believe on him to life everlasting.
 

Note: Actually Paul is saying that among sinners, he considered himself chief or in first place.  He was saying that he was the worse sinner.  Clearly this was his feeling and not fact.  All those called of God and receiving the Holy Spirit will think themselves a chief sinner at some time or another.  When we have our minds opened to our sin, we often or at times can feel as Paul did.  In fact, that is
what is being brought up in verse 16. 

In verse 16, Paul is saying that in the Salvation Process, Christ shows us mercy as He works with us to overcome and build His character in us.  Christ shows all longsuffering in this.  Why?  So that in Paul we (all firstfruits) see that "pattern" of Paul's, i.e. feeling like a chief sinner but being shown mercy by Christ and working with Him through the Salvation Process until He returns, at which time we receive eternal life.

Clearly, Paul was not the first called, the first forgiven, the first with the Holy Spirit or anything else.