Римлянам 1:21

Because that, when they knew God--that is, while still retaining some real knowledge of Him, and ere they sank down into the state next to be described. they glorified him not as God, neither were thankful--neither yielded the adoration due to Himself, nor rendered the gratitude which His beneficence demanded. but became vain--(compare Jer 2:5). in their imaginations--thoughts, notions, speculations, regarding God; compare Mat 15:19; Luk 2:35; Co1 3:20, Greek. and their foolish--"senseless," "stupid." heart--that is, their whole inner man. was darkened--How instructively is the downward progress of the human soul here traced!

In this chapter we may observe, I. The preface and introduction to the whole epistle, to Rom 1:16. II. A description of the deplorable condition of the Gentile world, which begins the proof of the doctrine of justification by faith, here laid down at Rom 1:17. The first is according to the then usual formality of a letter, but intermixed with very excellent and savoury expressions.

1:21 To know God in Scripture usually means to have an intimate, saving relationship with him (see 2 Cor 5:16; Gal 4:9; Phil 3:8, 10). Here, however, they knew God means that people knew about God. All people have some understanding of God through creation, yet they do not do what is right based on that knowledge. Rather than learn more about God, they worship gods of their own making.