Its original sense is ‘complete,’ and the corresponding verb τελειόω is to complete, or finish. The derived senses will vary with the subjects to which it applied.
(1.) In the Septuagint version of the Old Testament, it stands for sincere, upright, undivided in heart.
‘Noah was a just man, and perfect in his generations, and Noah walked with God.’ (Gen. vi. 9.)
Amaziah had a great deal of religion about him, but he was inconsistent; and, therefore, it says of him, ‘He did that which was right in the sight of the Lord, but not with a perfect heart.’ (2 Chron. xxv. 2.)
(2.) In Matt. v. 48, it means complete, or comprehensive, as opposed to being limited and partial.
Our Lord is exhorting His disciples to love their enemies as well as their friends (verses 43, 44); and says, ‘That ye may be the children of your Father which is in heaven: for He maketh His sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.’
He then goes on to show that there is no value in merely partial love (46, 47); and concludes with the words, ‘Be ye therefore perfect, even as your Father which is in heaven is perfect.’ That is, like Him; show your love to your opponents as well as to your brethren.
(3.) In Heb. v. 14, it is used of those who are ‘of full age,’ or complete in their growth, and therefore capable of strong meat; as contrasted with those who are babes, and therefore fed on milk. It is the same in Eph. iv. 13, 14; and 1 Cor. xiv. 20.
(4.) In 1 Cor. ii. 6, it means ‘fully initiated.’ A mystery was something not known to the world, but revealed to those who were initiated, and those who were initiated were called τέλειοι. So here St. Paul says, ‘We speak wisdom amongst them that are perfect,’ or the initiated: ‘yet not the wisdom of this world, nor of the princes of this world, that come to nought: but we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory.’
I am not sure whether Phil. iii. 15, expresses full initiation, or full manhood. I am inclined to think that it means initiation, as the verse speaks of a yet further manifestation of God’s will. ‘Let us therefore, as many as be perfect,’ or initiated, ‘be thus minded: and if in anything you be otherwise minded, God shall reveal even this unto you.’