SINCERE (εἰλικρίνεια, G1636, unadulterated, pure, unmixed; esp. in defining the right attitude to God and to His people). The symbolic OT laws forbade a great variety of mixtures (e.g., yoking the ox and ass together, planting together different seeds, Deut 22:9, 10), as an expression of the principle of holiness (separation, distinction). Israel was to be distinct from the nations as her God was from theirs. Especially Christ’s Sermon on the Mount develops this theme (“No one can serve two masters,” Matt 6:24). The NT doctrine in the epistles stresses the antithesis between new life in Christ and the old nature.
NT sincerity is an all-embracing attitude, not just one virtue among many. In Philippians 1:9, 10 it is the purity required at the Judgment, and requires growth in both love and knowledge, fitting one to discern that which is truly excellent. 2 Peter 3:11 also has the eschatological perspective in view; the sincere mind believes God’s promises of Christ’s return, and is not the attitude of the scoffers which covers up God’s promises with proud self-evaluation of the future. In 1 Corinthians sincerity is the unleavened bread (unmixed, plain) which rejects immorality in the church and the pride which condones it.
Sincerity has also application in ministry to others. Paul is not a peddler of the Word, but one who has only love as his motive (2 Cor 2:17), and he preaches not with earthly wisdom, but only from love; i.e., so he can be understood (2 Cor 1:12, 13). Singleness of motive and message go together.
International Standard Bible Encyclopedia (1915)
The Revised Version (British and American) has "sincere" for "pure" (2Pe 3:1), "sincerely" for "clearly" (Job 33:3).
In The Wisdom of Solomon 7:25 we have eilikrines in the description of Wisdom as a "pure influence," the Revised Version (British and American) "clear effluence."