Every sayer [says], "Allah is my Lord,"

yet if you ask them, they do not know Him.

And He Almighty pointed to this meaning by His saying in Surat al-A'raf:

{They said, "Have you come to us that we should worship God alone and leave what our fathers used to worship? Then bring us what you promise us, if you are of the truthful." He said, "Already have defilement and anger fallen upon you from your Lord. Do you dispute with me concerning names you have named, you and your fathers, for which God has sent down no authority?"} (Al-A'raf: 70-71).

And He Almighty said, pointing to this meaning in Surat Yusuf:

{O two companions of the prison, are diverse lords better or God, the One, the Prevailing? You worship not besides Him except names you have named, you and your fathers, for which God has sent down no authority. Command (al-Hukm) belongs to none but God. He has commanded that you worship not except Him. That is the correct religion, but most of the people do not know.} (Yusuf: 39-40).