A.T. Robertson Commentary Romans 1:19

"because that which is known of God is manifest in them; for God manifested it unto them." — Romans 1:19 (ASV)

Because (διοτ). Gives the reason (δια, οτ like our "for that") for the revelation of God's wrath.

That which may be known of God (το γνωστον του θεου). Verbal adjective from γινωσκω, either "the known" as elsewhere in N.T. (Acts 1:19; 15:18, etc.) or "the knowable" as usual in ancient Greek, that is "the knowledge" (η γνωσις) of God. See Php 3:8. Cf. same use of the verbal χρηστον in Ro 2:4, αμεταθετον in Heb 6:17.

Manifest in them (φανερον εν αυτοις). In their hearts and consciences.

God manifested (ο θεος εφανερωσεν). First aorist active indicative of φανεροω. Not mere tautology. See 2:14-16.