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Philippians 3:12 Updated American Standard Version (UASV)
12 Not that I have already obtained it or am already perfect [τελειόω teleioō], but I press on so that I may lay hold of that for which also I was laid hold of by Christ Jesus.
Philippians 3:15 Updated American Standard Version (UASV)
15 Let us therefore, as many as are perfect [τέλειος teleios], have this attitude; and if in anything you have a different attitude, God will reveal that also to you.
The Greek verb in 3:12 (τελειόω teleioō) means to be perfected, that is, to be in a perfect state with respect to a specific attribute, and here it means to succeed fully. The Greek-English Lexicon of the New Testament: Based on Semantic Domains rightly says, “It is important that in Php 3:12 τελειόω is to be understood in the sense of a functional stage of religious attainment.”
The Greek adjective in 3:15 (τέλειος teleios) literal means perfect, with the sense of mature, specifically spiritually mature. It is being at an advanced state of spiritual development, usually as a result of experience, teaching, and in most cases, time.
If we look at 3:12, Paul informs his readers that he did not see himself as being perfect at the time of his penning this letter. He wrote, “Not that I have already obtained it or am already perfect [teleios], but I press on …” These two verses are not contradicting each other. Being one of the most mature Christians of all time, Paul did not view himself as perfect, stating that he had not reached the ultimate goal, knowing that he had to continue to make advancements in his maturity in this imperfect age of humanity. So, we all the more so need to make advancements toward complete maturity. This is why Paul closed this section of chapter three out with, “Only to what we have attained, let us go on walking in the same.”– Philippians 3:16.
Paul likewise uses the word perfect in different ways. Some believers are mature or complete in their walk with the Lord. But no one is perfect in the sense of having fully arrived or reaching the ultimate goal, this will only happen at the “the resurrection from the dead” (3:11).
 Phroneo … signifies (a) “to think, to be minded in a certain way”; (b) “to think of, be mindful of.” It implies moral interest or reflection, not mere unreasoning opinion.―VCEDONTW
 Johannes P. Louw and Eugene Albert Nida, Greek-English Lexicon of the New Testament: Based on Semantic Domains (New York: United Bible Societies, 1996), 538.