I was going to post some of my notes on it but found the talk well-represented by the notes here.
I'll just post a few of the comments from that site:
Mohler then asked this: what is the urgency of this question? The answer is that there is great urgency in adequately addressing this question. There are some recent developments that indicate why this is so. The controversy concerning Bruce Waltke is just one example—Waltke said recently that unless we embrace evolution, evangelicalism will be reduced to the status of a cult. Meanwhile, we are constantly faced with supposed facts that science presents a challenge that must be embraced by the church. The current mental environment in which we live is an environment shaped by the intellectual assumption that the world is very old. To speak in confrontation to that environment comes at a significant cost. Even greater urgency is pressed upon us by the new atheism...Biblically this is where we must stand.
Mohler proceeded to argue for the theological necessity of understanding a young earth and 24-hour calendar days. He presented two great issues that arise when we allow for a day-age theory or any other old-earth understanding of creation.
The first issue concerns the integrity of Scripture. He conceded that many of those who hold to a day-age view are seeking to believe it without doing violence to the inerrancy of Scripture. And yet there are many issues that must be addressed. What is sorely lacking in the evangelical movement today is a consideration of the theological cost of such a view. This entire conversation is either missing or marginalized in the church today. The exegetical issues are real and the exegetical evidence based on a Reformation understanding of Scripture leads to a natural understanding of 24-hour days in creation. Mohler would allow that it might be possible that he is over-reading the text in this regard. For this reason we must hear the warnings of those who hold to an older view of the universe since it is possible that we may be creating an intellectual problem that is not necessary. And yet he simply finds that the exegetical cost and the theological cost is just too high.
An old-earth review raises at least two important issues. First, it raises the issue of the historicity of Adam. Paul bases his understanding of human sinfulness and Adam’s headship over the human race on a historical Adam and a historical Fall. An old earth understanding has serious complications because the old earth is not merely understood to be old but also through its age telling a story. The story it is telling is of billions of years of creation before the arrival of Adam. How then can it account for a historical Adam? An old earth understanding requires an arbitrary intervention of God in suddenly creating Adam and depositing him in the world. This presents problems both in Genesis and Romans.
The second question it raises regards the Fall. We understand from Genesis 3 and the entire narrative of Scripture that what we know in the world today as catastrophe, as natural disaster, as pain, death, violence, destruction, predation—that all of these are results of the Fall. We end up with enormous problems if we try to interpret a historical fall in an old-earth rendering. This is most clear when it comes to Adam’s sin. Was it true that, as Paul argues, when sin came, death came? Keep in mind that if the earth is old, and we determine it is old because of the scientific data, it also claims that long before the emergence of Adam there were all the effects of sin that are biblically attributed to the Fall. No Christian reading of the Scripture alone would ever come to this kind of conclusion. And once you come to such a conclusion it is very difficult to reconcile with the Bible. If the animosity between the lion and the lamb predates the Fall, what joy or purpose is there in saying that they will be reconciled in the consummation?
The avoidance of this question about the age of the universe will come at the cost of our own credibility. But disaster ensues when the book of natural revelation is used to trump the book of special revelation. We would not be having this discussion today if these questions were not being posed to us by those who assume that general revelation is providing to us compelling evidence that forces us to reconstruct our understanding of the biblical text, that the assured results of science are forcing us to rethink what the Bible seems to say. Great caution is in order when we begin to cede to science. The assured results of science—what do they tell us about a virgin birth? About a resurrection? About sexual orientation? Are we going to submit special revelation to what science says in all of these areas? The end of this process is theological disaster.
When it comes to the confrontation of evolutionary theory and the gospel we have a head-on collision. It is our responsibility to give an answer to this question of why the universe looks old, but the most natural understanding comes to this: the universe looks old because the Creator made it whole. When he made Adam, Adam was not a fetus but a man. By our understanding this would have required time. But for God it did not. He put Adam in the garden, which was not merely seeds, but a fertile, mature garden. God creates and makes things whole. And secondly, it looks old because it bears the effects of sin, the flood, catastrophe. Creation is groaning and in its groaning it looks old and worn, giving us empirical evidence of the reality of sin.
In the end the conclusive answer to this question is known only to God. This is where we are left; and it is a safe place to be.
But it would also help if we could demonstrate some solid scientific proofs. Trouble is I think we've demonstrated many already but the kind of theory we're dealing with is nothing but a complex convoluted self-validating myth that can morph into anything they need it to be to answer us.
AND of course, face it, too many creationists do come up with some untenable stuff.