Paul called his personal achievements (Philippians 3:4-6) as valuable as dung (v. 8), rejecting his own righteousness (v. 9). Now he is focused on reaching the "high calling of God in Christ Jesus." He is well aware that he has not already attained God's ultimate design for him, nor is he perfect in any sense of what he will become, but he intends to follow after and be "apprehended" of the Lord Jesus (Philippians 3:12).
To begin with, Paul knows that he must forget "those things which are behind." Not only his rather stellar reputation and achievements, but especially the awful conditions of being "without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world" (Ephesians 2:12). Those terrible conditions were done away with when Paul (and those of us who are twice-born) were created after God "in righteousness and true holiness" (Ephesians 4:24).
Now, having been apprehended of Christ and forgetting those things that were part of our old lives, we can "press toward the mark for the prize of the high calling of God in Christ Jesus" (today's verse). Like for an athelete, the prize is gained only by those who win, not by those who run "uncertainly" (1 Corinthians 9:24, 26).
Thus, we cannot win if we have "two masters" (Luke 16:13), nor can we please "him who has chosen" us if we entangle ourselves with the affairs of this life (2 Timothy 2:4). May God keep us focused on the prize. HMM III
http://www.icr.org/article/following-mark/