To my understanding there are mainly two aspects of the significance of the second part of the Shahadah, namely: “Muhammad is the Messenger (Prophet) of Allah”.
First: Allah Almighty is the Transcendental Creator and Sustainer of the universe, Who does not ‘materialize’ or appear before us. We know about Allah as well as about His guidance through the Messenger Allah Himself chose. This applies to all the Messengers of Allah, not only to Muhammad (peace be upon him).
But, Muhammad (peace be upon him) is the Final Messenger of Allah who came to coordinate, confirm and complete the earlier messages. So he is not just one messenger among many; rather he represents all the Messengers of Allah before him. Actually Muhammad fulfils and revalidates all the earlier messages through his mission. Thus, he stands for the completion of Allah’s guidance for mankind.
Second: God’s guidance is not just for our academic study, but for our realizing it in space and time. The role of Prophet Muhammad is not merely to teach us, but to practically demonstrate to us how we can live by God’s guidance. This means that the Qur’an is, as it were, the ‘theory’; and the Sunnah (the example of Prophet Muhammad) is the ‘practical form’ of that theory.
Islam affirms and underscores the importance of the Qur’an (the precept) and the Sunnah (the example) together. This points to the fact that it is not enough for us just to ‘worship’ God as an ascetic devotee would; but also to live here as members of human society, making use of the blessings God has given us, and observing the limits set by God. For this purpose, we need a practical model who would not only convey God’s message to us, but also LIVE that message here below, as God wants us to live it.
In short the two parts of the Shahadah are like the two sides of the same coin. One part states one’s acceptance of the One True God, which relates to one’s faith and conviction, which is inward. The other states one’s acceptance of God’s Messenger on earth, which relates to one’s commitment to live in obedience to God’s commandments, which expresses itself in one’s outward life as lived daily.
That is why Allah in the Noble Qur’an commands:
“You who believe! Obey Allah and His Apostle” (Al-Anfal 8:20)
“Say: “If ye do love Allah, Follow me: Allah will love you and forgive you your sins …” (Aal `Imran 3:31)
“He who obeys the Apostle, obeys Allah…” (An-Nisaa’ 4:80)
These verses make clear that the acceptance of the Prophet is the acceptance of Allah. This is because it is the Prophet who received the Noble Qur’an and brought it then to people. So, any person who questions the Prophet, is actually questioning Allah and His Qur’an.
Hence, the affirmation of the prophethood of Muhammad (peace be upon him) is the natural corollary of the declaration of faith in Allah. In other words, the second part of the Shahadah derives its significance from the first; so it becomes incumbent on any one who accepts the first part of the Shahadah, to accept the second part too.
And Allah knows best.
(Answer provided by Shahul Hameed in onIslam.net)