First of all, that's not a good practice. The best way, in my opnion, would be to have an code behind that uses your class, get the message and set it to the html.
But, if you really want to do it...
If you just want to write something exatly after the last thing was wrote, use: System.Web.HttpContext.Current.Response.Write("My Text")
// Set a timeout of 1 second, in case the script is added at the beging of the page
// and the DOM is not ready yet.
document.getElementById("YourElementId").innerHTML = "Your Text";
It seems to me that you simply want to display correct? Not sure why you are using response.write. Don't you have any controls on your .aspx page that you can access? For example, if you have a label control, from your code behind, you can access its text property to display information. For example...