Allah told about the Jews that they knew Muhammad was a prophet in Surah al-Baqarah, 146:
يَعْرِفُونَهُ كَمَا يَعْرِفُونَ أَبْنَاءَهُمْ
which means: [Those whom we have given the book recognize him as they recognize their son.] However, they did not accept, so they belie him with their tongues while they know he is a prophet. The Torah revealed to Moses contained the news of Muhammad as the Messenger of Allah. However, the Torah and Injil have been perverted textually after their meanings were perverted. Muhammad was sent to the entire world of humans and jinn. The world here refers to the humans and jinns as in Surah al-Furqan 1:
لِيَكُونَ لِلْعَالَمِينَ نَذِيرًا
This means he (Prophet Muhammad) was sent to all humans–Arabs and non-Arabs–and to the entire jinn.
He is truthful in all what he conveys from Allah, so that they believe in the rules he brought and follow him.
It is obligatory to believe our master Muhammad is truthful in all what he came with, whether the news of future happenings like the Hereafter, or the news of previous nations, or legitimizing a matter or forbidding it.
What is meant by the two testifications is negating the status of Godhood for others and confirming it to Allah, the Exalted,
This statement negates that anything other than Allah deserves to be worshipped; It also confirms Allah alone deserves to be worshipped, while having the conviction about the message of our master Muhammad
That is, while admitting and believing in the message of Muhammad
Allah, the Exalted, said in Surah al-Fath, 13:
وَمَنْ لَمْ يُؤْمِنْ بِاللَّهِ وَرَسُولِهِ فَإِنَّا أَعْتَدْنَا لِلْكَافِرِينَ سَعِيرًا(13)
which means: [And whoever does not believe in Allah and His Messenger, then surely We have prepared a fervid fire for the blasphemers.]
This verse proves that believing in Muhammad is a must for ones belief to be valid with Allah; The one who doubts or denies that is judged as a blasphemer for belying the Qur’an. Likewise, this verse gives the meaning the one who believes in Allah and His Messenger and does not perform any of the obligations is not a blasphemer, and shall not be everlastingly in Hellfire.
Hence, this verse is explicit in judging a blasphemer one who does not believe in Muhammad. The one who denies this matter is in fact belying the Qur’an; and the one who belies the Qur’an is a blasphemer.
The one who denies this matter thereby denying the belief in Muhammad is a blasphemer.
The Muslim scholars hold the consensus on judging as blasphemer the one who does not embrace Islam;
This means, anyone who embraces a religion other than Islam is a blasphemer.
And on judging a blasphemer, the one who does not pass the same judgment, doubts about it, or wavers, as to say, “I do not say whether or not he is a blasphemer.”
The one who hesitates in judging someone as a blasphemer for not embracing Islam but instead embraces Judaism, or other is judged as a blasphemer. Likewise, is judged as a blasphemer, if he says “he might or might not be a blasphemer.” Even though the one who says such a statement might claim (by his tongue) to be a Muslim, still he is judged as a blasphemer and doubting and stopping in judging him as a blasphemer is blasphemy by itself.
Know, with certitude, ones belief –Islam–is not valid and ones good deeds are not accepted without uttering the Testification of Faith by saying:
أشهد أن لا إله إلّا الله
وأشهد أنّ محمَّدًا رسول الله
I testify no one is God except Allah and I testify Muhammad is the messenger of Allah,” or by saying what has the same meaning even in other than the Arabic language. It is sufficient for the validity of ones Islam to utter once in a lifetime, however uttering it in every prayer it remains obligatory for the validity of the prayer; this is applicable to the non-Muslim who wants to embrace Islam;