I think it is more accurate to say that the unbelievers "see" what God
"makes plain" to them, but refuse to acknowledge the truth of what they
see. This refusal is a moral failure, rooted in sin (as George Murphy
rightly points out), and not an epistemic failure.
But George has often expressed what I take to be a denial of the legitimacy
of natural theology, and it is that which I want to comment on. I do not
regard natural theology as in anyway capable of giving salvific knowledge
of God--that is related to the special revelation delivered in the manger
and culminating at the cross and the resurrection--but it is not valueless.
Natural revelation can serve a distinct function in evangelism (call it
pre-evangelism or apologetics, as is common). My aim is not to see people
converted to theism but to Christ, but for many, they must first be willing
to acknowledge the theistic hypothesis before they will consider the claims
of Christ. That the Holy Spirit can use natural theology, producing a
general theistic belief, as a stage in prevenient grace, seems clear from
e.g. Paul's Mars Hill dialogue.
Wishing all a Christmas season filled with joy and grace,