Romans 1:21

because that, when they knew God i.e. as primevally revealed, and then constantly witnessed to by the visible Creation as Eternal and Omnipotent. "To know God" is a phrase capable of many degrees of meaning, from the rational certainty of a Supreme Personal Maker and Lord up to that holy intimacy of divinely-given communion with the Father and the Son, to which the words of John 17:3 refer. In this passage all that is necessary to understand is the certainty (however learnt) of the existence of a Personal Omnipotent Creator.

they glorified him not as God, neither were thankful The verbs throughout this passage are aorists. The processof declension from the truth is not dwelt upon, so much as the factthat it did take place, at whatever rate. There was a time when man, although knowledge of God had been given him, ceased to praise Him and to thank Him for His "great glory" and His rich gifts; turning the praise and thanks towards idol-objects instead. We must note that these first marks of decline (failure to praise and to thank Him), indicate a subtle and lasting secret of idolatry. Man, conscious of guilt before the Eternal, shrinks from directworship. In mistaken reverence, it may be, he turns away to "the Creature," to address his praises there. But the result is inevitable; the God unworshipped rapidly becomes unknown.

but became vain in their imaginations "Vain," here, as often in Scripture, is "wrong," morally as well as mentally. "Imaginations" is rather thinkings: the Gr. is a word often rendered "thoughts," (as e.g. Matthew 15:19.) In Philippians 2:14 it is rendered "disputings;" in 1 Timothy 2:8, "doubting." The verb is used in e.g. Luke 12:17, for the balancing of thing against thing in the mind. Both verb and noun, when the context gives them an unfavourable reference, indicate a habit of captious and hesitating thought such as would ignore plain testimony and attend to abstract difficulties by preference. Thus here, man, growing unused to adoration of his God, fell to independent thinking, (in however rude a form,) and "in" this, occupied in this, "became vain," went astray altogether.

their foolish heart "Foolish," more strictly unintelligent; failing to see connexions and consequences. Same word as Matthew 15:16. The "heart" may here mean merely the intellect, as perhaps in Mark 2:6; Mark 2:8. It is almost always difficult, however, to trace in Scripture (as indeed so often in constant experience) the borderbetween reason and conscience. "Heart" certainly includes both in the majority of N. T. passages.