12 Not as though I had already attained, either were already perfect: but I follow after, if that I may apprehend that for which also I am apprehended of Christ Jesus.
13 Brethren, I count not myself to have apprehended: but this one thing I do, forgetting those things which are behind, and reaching forth unto those things which are before,
14 I press toward the mark for the prize of the high calling of God in Christ Jesus.
15 ¶ Let us therefore, as many as be perfect, be thus minded: and if in any thing ye be otherwise minded, God shall reveal even this unto you.
16 Nevertheless, whereto we have already attained, let us walk by the same rule, let us mind the same thing.
My understanding is that God has given every man a measure of faith and grace (Rom. 12:4,7) and Christian growth is a process of maturing.
When he says, "whereto we have already attained" his is talking about where we are currently at in our Christian maturity.
When he says, "let us walk by the same rule" his referring to the rule set forth in verses 12-14.
When he says, "let us mind the same thing" he is referring to verse 15 where the term "minded" is introduced. Those who have reached a mature status (little children, young men, fathers) realize they are not perfect but need to press on to the mark and forgett the failures as they press forward.
That is my take on it. Nothing technical just a general commentary. I hope to be growing throughout my Christian life and growth includes correction and progress, so I am not above correction either.