“This doctrine of the Total Depravity of man makes it plain that the moral consciousness of man as he is today cannot be the source of information about what is ideal
good or about what is the standard of the good or about what is the true nature of the will which is to strive for the good. It would seem plain enough that men have to choose on this point between the Christian and the non-Christian position.
It is this point particularly that makes it necessary for the Christian to maintain without any apology and without any concession that it is Scripture, and Scripture alone, in the light of which all moral questions must be answered. Scripture as an external revelation became necessary because of the sin of man. No man living can even put the moral problem as he ought to put it, or ask the moral questions as he ought to ask them, unless he does so in the light of Scripture. Man cannot of himself truly face the moral question, let alone answer it.” Cornelius Van Til, “The Defense of the Faith” pg. 77
Feileadh Mor 
That very doctrine is also what makes it important to oppose Christianity at every opportunity.
I think Van Til’s biggest legacy was the shift in the paradigm of apologetics, of having our theology (or should I say Reformed Theology) shape our apologetic methodology. Thanks for this quote.