Short answer: Proverbs 21:21 most likely comes from Solomon—or from the Solomonic wisdom tradition—according to the way evangelical Christians have traditionally read and handled the book of Proverbs.

Key points an evangelical Christian would emphasize

– Traditional attribution: Proverbs 1:1 explicitly names Solomon: “The proverbs of Solomon son of David, king of Israel.” Evangelicals usually take this as indicating that the bulk of the sayings in Proverbs (including chap. 21) reflect Solomon’s teaching or the Solomonic school of wisdom.

– Book structure and authorship nuance: The book is a collection. While Proverbs 1–24 and 25–29 are largely associated with Solomon (25–29 are introduced as “more proverbs of Solomon copied by the men of Hezekiah,” Prov. 25:1), chapters 30 and 31 are explicitly credited to Agur and King Lemuel. Evangelicals therefore affirm Solomon as the most likely author or originating source for most proverbs in the book, while also recognizing that some sayings may have been collected, edited, or preserved by others over time.

– Historical background: Solomon was the son of David and king of Israel in the 10th century BC. The biblical portrait stresses that God gave Solomon special wisdom (1 Kings 3), and Jewish and Christian tradition has long credited him with compiling proverbs and other wisdom literature (Ecclesiastes and Song of Songs are also traditionally linked to him).

– Inspiration and authority: From an evangelical perspective, regardless of the human means of composition and compilation, Proverbs is part of the God-breathed Scripture (2 Tim. 3:16). Thus Solomon’s proverbs are treated as inspired wisdom for teaching, rebuking, correcting, and training in righteousness.

– Genre and purpose: Proverbs is wisdom literature—practical, didactic sayings intended to form godly character. Evangelicals read Proverbs as normative moral and spiritual guidance, not merely cultural maxims.

Brief textual note on Proverbs 21:21 itself
– Common translation (NIV): “Whoever pursues righteousness and love finds life, prosperity and honor.” The verse fits the book’s theme: the pursuit of righteousness (right conduct before God and neighbor) and steadfast love/mercy brings life and blessing. Evangelicals often connect this teaching with the larger biblical commands to live justly and love mercy (cf. Micah 6:8; Jesus’ summary of the law to love God and neighbor).

Practical evangelical application
– Solomon’s wisdom is read as pointing people to fear the Lord as the foundation of wisdom, while also calling Christians to active pursuit of righteousness and love, trusting God’s blessing and transformation rather than relying on mere external behavior.

If you’d like, I can give a brief commentary on Proverbs 21:21 from a conservative evangelical commentator (e.g., Kidner or others) or suggest sermon/application points based on the verse.