I got it to work, with some modifications:

Sub Testsub()
Dim Sheet, Sheet_ As Worksheet, I As Integer
Set Sheet = ActiveWorkbook.Sheets(1)
MsgBox ActiveWorkbook.Sheets.Count
For I = 1 To ActiveWorkbook.Sheets.Count
Set Sheet_ = ActiveWorkbook.Sheets(I)
If Sheet_.Name < Sheet.Name Then Sheet_.Visible = False
Next
Sheet.Activate
End Sub