Romans 1:19

Because that which may be known of God
There are some things which could not be known of God by the light of nature; as a trinity of persons in the Godhead; the knowledge of God in Christ as Mediator; the God-man and Mediator Jesus Christ; his incarnation, sufferings, death, and resurrection; the will of God to save sinners by a crucified Jesus; the several peculiar doctrines of the Gospel, particularly the resurrection of the dead, and the manner of worshipping of God with acceptance: but then there are some things which may be known of God, without a revelation. Adam had a perfect knowledge of him; and his sons, though fallen, even the very Heathens have some notion of him, as that there is a God; and by the light of nature it might be known that there is but one God, who is glorious, full of majesty, and possessed of all perfections, as that he is all powerful, wise, good and righteous: and this

is manifest in them,
or "to them"; by the light that is given them: it is light by which that which may be known of God is manifest; and this is the light of nature, which every man has that comes into the world; and this is internal, it is in him, in his mind and conscience, and is communicated to him by God, and that by infusion or inspiration; see ( Job 32:8 ) ;

for God hath showed [it] unto them;
what may be known of him by that light; and which is assisted and may be improved by a consideration of the works of creation and Providence.