Thank you for your question. Although a person should strive to reach personal satisfaction in the truth of the usul al-din, be that through logical thinking or through religious experience, having not passed through that process does not make a person a kafir. As soon as a person utters the shahadatayn they are a Muslim and it is only through renouncing their faith that they fall into kufr. Rather, a deep comprehension of these principles is something that takes time, and a person is a Muslim throughout that journey.
May you always be successful.