The Meaning of the Two Testifications of Faith
In general, the meaning of the Testification that “no one is God except Allah” is: I confess with my tongue and I hold as a conviction and submit to it in my heart that the only rightfully worshipped One is Allah the Exalted.
The meaning of the Testification that “Muhammad is the Messenger of Allah” is: I confess with my tongue and I accept with my heart that our Master Muhammad ﷺ is sent from Allah to all of the humans and jinn. Muhammad ﷺ was truthful in everything he conveyed from Allah the Exalted, therefore the people must believe in his Laws and follow him.
What is meant by the two Testifications of Faith is the negation of Godhood for other than Allah and confirming it for Allah the Exalted, along with the confession of the Message of our Master Muhammad ﷺ. Allah the Exalted Said:
﴿وَمَن لَّـم يُؤمِن بِٱللهِ وَرَسُولِهِ فَإِنَّآ أَعتَدنَا لِلكَــٰفِرِينَ سَعِيرًا﴾
[which means] “Whoever does not believe in Allah and His Messenger, then he is a blasphemer, and We have Prepared Hell for the blasphemers.” This verse is explicit in charging whoever does not believe in Muhammad ﷺ with blasphemy, and whoever argues with this point would be objecting to the Qur’an, and whoever objects to the Qur’an blasphemes.
The Islamic scholars have agreed on charging whoever practices a religion other than Islam with blasphemy, as well as whoever does not charge him with blasphemy, or doubts, or remains neutral [about his blasphemy]; like for one to say “I do not say he is a blasphemer and I do not say he is not a blasphemer.”
Know with certainty that Iman (faith) and Islam would not be valid, nor would the good deeds be accepted, without the two Testifications of Faith, i.e., by the expression: “I testify that no one is God except Allah and I testify that Muhammad is the Messenger of Allah,” or by whatever carries the meaning of these two statements, even if it were expressed in other than the Arabic language. It is enough for the validity of one’s Islam to utter it once in a lifetime. After that, the obligation of saying the Testification of Faith remains for the validity of every prayer. This is in reference to the one who was following a religion other than Islam, then he wanted to enter Islam. In reference to the one who was raised as a Muslim and he believed in the two Testifications of Faith, uttering them is not a condition. Rather, he is a Muslim even if he does not utter.
The Messenger of Allah ﷺ said:
قَالَ اللهُ تَعالى: وَمَا تَقَرَّبَ إِلىَّ عَبْدِي بِشَىءٍ
أَحَبَّ إِلىَّ مِمَّا افْتَرَضْتُ عَلَيْهِ
[which means] “Allah Said, ‘My slave does not come closer to Me (i.e. he does not gain a higher status from Me) by doing anything more acceptable to Me than what I have Made obligatory upon him’.” This is a Qudsiyy hadith narrated by Al-Bukhariyy. Moreover, the best and first obligation is the belief in Allah and His Messenger.
The Creed that no one is God except Allah alone is not enough if it is not associated with the Creed that Muhammad ﷺ is the Messenger of Allah. Allah the Exalted Said:
﴿قُل أَطِيعُواْ ٱللهَ وَٱلرَّسُول فَإِن تَوَلَّوا فَإِنَّ ٱللهَ لَا يُحِبُّ ٱلكَــٰفِرِينَ﴾
[which means] “(O Muhammad,) say, ‘Obey Allah and the Messenger,’ and if they turn away (from believing in Allah and His Messenger) then Allah does not Accept the blasphemers.” This means that because of his blasphemy, Allah does not Accept the one who turned away from believing in Allah and His Messenger, and what is meant by “obeying Allah and His Messenger” in this verse is “believing in them.”
This is evidence that whoever does not believe in Allah and His Messenger, Muhammad ﷺ, is a blasphemer, and Allah does not Accept him because of his blasphemy. Whoever says that Allah loves the believers and the blasphemers because He Created all of them has contradicted the Qur’an. So it is said to him, “Allah Created all of them, but He does not Accept all of them.”
 Surat Al-Fath, ayah 13.
 Surat Al ^Imran, ayah 32.