Philippians 3:13

Textus Receptus:
αδελφοι εγω εμαυτον ου λογιζομαι κατειληφεναι εν δε τα μεν οπισω επιλανθανομενος τοις δε εμπροσθεν επεκτεινομενος

King James:
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,

Comments:
count not myself to have apprehended: As Paul reckons ("count" here is the word "reckon" in Romans 8:18) he has not yet "laid hold on eternal life" (to borrow the wording of 1 Timothy 6:12.) As in that passage, his honest reckoning reveals both that he securely belongs to Christ (with the future implied by that belonging) and that his current state in this world (both in terms of persecution and personal failure) does not fully reveal that belonging.

this one thing I do: The one thing he does is in the next verse. The two participles ("forgetting" and "reaching forth") only modify the one thing he does--press toward the mark.

forgetting: Paul cannot mean by forgetting that he is unable to recollect his past. He recalls that very past in the opening verses of this chapter (verses 4-7, where he names the thing he has now disregarded.) Instead, he means that he has chosen not to hold those things important any more--just as God can speak of "forgetting" Judah in passages like Jeremiah 23:39.

reaching forth: Before Paul names the one thing he does, he puts it in a temporal context using the analogy of a race. He disregards the past and ardently anticipates the future. But the activity that marks his Christianity in this world is in the next verse.

Expanded Paraphrase:
You share my condition as my brothers in Christ. I do not reason through and conclude that I have already achieved my perfection. But I do relegate to insignificance what used to be most important to me. And I do diligently look forward to my future fulfillment. No, more. While I do the one thing that makes a difference here and now I refuse to look back (as if it would help to return to the starting line) and I stretch out desperately toward the finish line (as if getting there is my only goal.)