The main thing is not just that you worship Allah, but that you also reject all worship directed towards others besides Allah.

The Messenger of Allah said:

“Worship Allah and give Him no partners.” [Reported by Al Tirmidhi, who deemed it good and authentic]

There is evidence in this that some people worship Allah while giving partners to Him. So, simply worshiping Allah is not enough to be considered a monotheist. It may be that the person worships Allah and something else at the same time. And this is well known:

“And most of them believe in Allah only while they associate others with Him.” [Surah 12, v.106]

The Muslim must be cautious about this point. The main thing is not just that you worship Allah, but that you also reject all worship directed towards others besides Allah.

Source: “Explanation of the Forty Hadiths of Al-Nawawi”, by Sheikh Moustapha Mabram, may Allah preserve him.