Natural theology is the knowledge of God that can be arrived at through general revelation, independent of the Bible and the person and work of Jesus Christ. Classical medieval theology, following Aquinas, had a high view of human reason. It therefore accorded natural theology an independent status, co-ordinate with Scripture. 
But this does not take sufficient account of the noetic effects of sin. All people do have knowledge of God; but this ‘natural’ knowledge does not bring them into relationship with him, but only condemns them. Natural philosophy and natural worship, independent of Biblical revelation, inevitable rejects God in favour of idols. This is the burden of Rom 1:18-3:20. Paul's Areopagus speech in Acts 17 on the one hand affirms that the Athenians do know something of God, but is quite stinging in his criticism of their idolatry.
God never intended creation to be sufficient for a relational knowledge of him; that has always required a covenantal word. Even in the Garden of Eden, God didn't leave Adam & Eve to empirically observe the Garden and say "wow - what a cool place - God must really love us!"; God spoke to them. Relational knowledge of God has always required a revelatory, covenantal word.
Furthermore, sinful humanity suppresses knowledge of God that they have. This rebelliousness can only be corrected by God’s Spirit, working through the Word. Natural theology has always had an incipient dualism. By prioritising "rational debate", it assumes there is a realm of "reason" that is free from sin.
The Reformers made natural theology dependent upon revelation. They demoted it from being an independent source of knowing God, to be part of Christian, Biblical doctrine. This manoeuvre is sometimes termed a ‘theology of nature’ rather than a ‘natural theology’. Such a theology still provides rich resources for life, worship, apologetics and evangelism. A regenerate mind can see how God uses creation to mediate elements of salvation. In Christ, the holy God re-sanctifies this fallen creation, so that it can fulfil its original doxological purposes. The gospel points to the God of order re-ordering his disordered creation. Seen through the "spectacles of scripture", creation is the "theatre of his glory" (Calvin).
A theology of nature provides many "points of contact" between the gospel and world. The incarnation demonstrates that a normal human body is a fit receptacle for divinity. The phenomenon of prophecy, and the existence of the written word, demonstrate that ordinary human language is a fit receptacle for the word of God. The sacraments of baptism & the lord's supper are effective because God uses ordinary physical elements - water, bread, grape juice - as effective signs to strengthen faith in him. The Bible makes sense of science's discovery that all of life is ordered and organically inter-dependence. More deeply - the ordered, life-giving God of the Bible provides the fundamental basis for the scientific presupposition that nature is instrumentally observable, and that experiments are replicable.
Demoting natural theology from being an independent source of the knowledge of God, and making it dependent upon revelation - a "theology of nature" - seems to keep all the benefits of providing "points of contact" between the gospel and the world, while avoiding the problems of dualism.
But this does not take sufficient account of the noetic effects of sin. All people do have knowledge of God; but this ‘natural’ knowledge does not bring them into relationship with him, but only condemns them. Natural philosophy and natural worship, independent of Biblical revelation, inevitable rejects God in favour of idols. This is the burden of Rom 1:18-3:20. Paul's Areopagus speech in Acts 17 on the one hand affirms that the Athenians do know something of God, but is quite stinging in his criticism of their idolatry.
God never intended creation to be sufficient for a relational knowledge of him; that has always required a covenantal word. Even in the Garden of Eden, God didn't leave Adam & Eve to empirically observe the Garden and say "wow - what a cool place - God must really love us!"; God spoke to them. Relational knowledge of God has always required a revelatory, covenantal word.
Furthermore, sinful humanity suppresses knowledge of God that they have. This rebelliousness can only be corrected by God’s Spirit, working through the Word. Natural theology has always had an incipient dualism. By prioritising "rational debate", it assumes there is a realm of "reason" that is free from sin.
The Reformers made natural theology dependent upon revelation. They demoted it from being an independent source of knowing God, to be part of Christian, Biblical doctrine. This manoeuvre is sometimes termed a ‘theology of nature’ rather than a ‘natural theology’. Such a theology still provides rich resources for life, worship, apologetics and evangelism. A regenerate mind can see how God uses creation to mediate elements of salvation. In Christ, the holy God re-sanctifies this fallen creation, so that it can fulfil its original doxological purposes. The gospel points to the God of order re-ordering his disordered creation. Seen through the "spectacles of scripture", creation is the "theatre of his glory" (Calvin).A theology of nature provides many "points of contact" between the gospel and world. The incarnation demonstrates that a normal human body is a fit receptacle for divinity. The phenomenon of prophecy, and the existence of the written word, demonstrate that ordinary human language is a fit receptacle for the word of God. The sacraments of baptism & the lord's supper are effective because God uses ordinary physical elements - water, bread, grape juice - as effective signs to strengthen faith in him. The Bible makes sense of science's discovery that all of life is ordered and organically inter-dependence. More deeply - the ordered, life-giving God of the Bible provides the fundamental basis for the scientific presupposition that nature is instrumentally observable, and that experiments are replicable.
Demoting natural theology from being an independent source of the knowledge of God, and making it dependent upon revelation - a "theology of nature" - seems to keep all the benefits of providing "points of contact" between the gospel and the world, while avoiding the problems of dualism.
Comments
One of the interesting things you encounter when you start reading non-evangelical writers is how much they value general revelation.
I remember reading the response of David Milikan to Peter Jensen's Boyer lectures. From what I can gather, David's relative conservative by Uniting standards. But I remember his comment that PFJ's lectures made no mention of general revelation, and how listening to Shoskanovich (sorry, I know that I'm misspelling his name) brought him as close to God as reading his Bible did.