. , .
Sub Sample()
Dim a As Variant
Dim s As String
Dim strTemp As String
s = "CC"
strTemp = "A,B,C,D"
a = Split(strTemp, ",")
Debug.Print IsInArray(s, a)
End Sub
Function IsInArray(stringToBeFound As String, arr As Variant) As Boolean
IsInArray = (UBound(Filter(arr, stringToBeFound)) > -1)
End Function

, , Runtime error '9': Subscript out of range. , Excel 2011?
Sub Sample()
Dim a As Variant
Dim s As String
Dim strTemp As String
s = "CC"
strTemp = "A,B,C,D"
a = Split(strTemp, ",")
Debug.Print IsInArray(s, a)
s = "A"
Debug.Print IsInArray(s, a)
End Sub

. . .
Sub Sample()
Dim a As Variant
Dim s As String
Dim strTemp As String
s = "CC"
strTemp = "A,B,C,D"
a = Split(strTemp, ",")
Debug.Print IsInArray(s, a)
s = "A"
a = Split(strTemp, ",")
Debug.Print IsInArray(s, a)
End Sub
Function IsInArray(stringToBeFound As String, arr As Variant) As Boolean
IsInArray = (UBound(Filter(arr, stringToBeFound)) > -1)
End Function
