No match for 'operator==' in function

I am working on a caesar decipher project, and in the CaesarDecipher function I keep getting this error when compiling:

error: no match for ‘operator==’ in ‘textInit.std::basic_string<_CharT, _Traits, >_Alloc>::operator[] [with _CharT = char, _Traits = std::char_traits, _Alloc >= std::allocator](((long unsigned int)i)) == alphabet[j]’

Here is the code for that function:

string CipherMessage::CaesarDecipher(string key)
{
  int keyValue;
  int charValue;
  string textInit = m_text;
  string textFinal;
  // Initializes an array containing the alphabet. A=index 0, B=index 1, etc
  string alphabet[26] = {"A","B","C","D","E","F","G","H","I","J","K","L","M","N","\
O","P","Q","R","S","T","U","V","W","X","Y","Z"};

  for (int i=0; i<=25; i++){
    if (alphabet[i] == key)
      keyValue = i;
  }
for (int i=0; i<=textInit.length(); i++){
    for (int j=0; j<=25; j++){
      if (textInit[i] == alphabet[j])   // Error occurs here
        charValue = j;
    }
    charValue = (charValue+keyValue)%26;
    for (int j=0; j<=25; j++){
      if (charValue == j)
        textFinal += alphabet[j];
    }
  }
  cout << "Final " << textFinal << endl;

  return textFinal;
}

Can anyone help?