The subject of توحید Tauhid (Oneness of Allah) which began with the opening of Surah 'Al-` Imran reappears in the first verse in a very special manner. Out of the three witnesses mentioned here, the evidence of Allah Almighty is in a figurative sense. It means that the being and the attributes of Allah and the manifestations of His power and creativity all over are open signs of His Oneness. Then there are the Messengers and Books sent by Allah, all confirming. that truth. The second testimony mentioned is that of angels who are close to Allah. They carry out His instructions and bear testimony (knowing and seeing) that Allah alone is worthy of worship.
The third witness is that of the men of knowledge. This expression, ulul` ilm, means the blessed prophets and men who have the knowledge of Islam. Imam al-Ghazali and Ibn Kathir رحمۃ اللہ علیہم find this a matter of great honour for ` Ulama for Allah has mentioned their witness along with His, and that of angels. It is also possible that the term, ulul` ilm (the men of knowledge) may, in an absolute sense, be referring to those who, by conducting an inquiry based on sound intellectual principles, or by deliberating into this universe, could acquire the knowl-edge of the unity of Allah Almighty - even if they are not ` alim or 'men of knowledge' in the traditional sense, whose knowledge is acquired under a formal discipline from Islam's own original sources. The next verse (19) clearly settles that the only faith' (دین din) acceptable to Allah is Islam. Thus, totally excluding any other faith or religion as acceptable to Allah, the verse completes the subject of Allah's Oneness, any opposition to which is doomed.
Related considerations
Merits of the verse meaning "Allah bears witness"
The verse beginning with شَهِدَ اللَّـهُ : 'shahidal-lahhu' has a special elegance. Imam al-Baghawi (رح) ، the renowned mufassir reports that two leading Jewish scholars came to Madinah from Syria. With the township of Madinah before them, they started talking to each other about the looks of the place which matched the prophecy in Torah that the last of the prophets would be living here. Later, they came to know that someone very pious lives here and whom people refer to as the prophet. They went to see the Holy Prophet ﷺ sight of him reminded them of all attributes which Torah had predicted he would have. They presented themselves before him and said: "You are Muhammad?" He said: "Yes." Again, they said: "You are Ahmad?" He said: "Yes, I am Muhammad and Ahmad." Then they said: We are going to ask you a question. If you answer it correctly, we shall embrace Islam." He ﷺ said: "Go ahead and ask." They asked: "Which is the greatest witness in the Book of Allah?" This verse of شَهِدَہ 'shahadah' (witness) was revealed as an answer to this question. He recited it for them. Both of them embraced Islam immediately.
According to a hadith in the Musnad of Ahmad, when the Holy Prophet ﷺ recited this verse in ` Arafat, he said right after that:
و انا علیٰ ذلک من الشٰھدین یا رَبِّ
And to that, 0 my Lord, I too am a witness. (Ibn Kathir)
And a narration from Imam al-A'mash (رح) says: Whoever recites this verse and then says: و انا علیٰ ذلک من الشٰھدین یا رَبِّ ; (And to that, 0 my Lord, I too am a witness), Allah Almighty will say to the angels on the Last Day: 'My servant has made a promise while I am the Foremost to keep a promise when I make it, so let my servant be admitted into Paradise.' (Ibn Kathir)
Another hadith from Sayyidna Abu Ayyub al-Ansari ؓ narrates that the Holy Prophet ﷺ said: 'Whoever recites Ayah al-Kursi (2:255) and the Ayah : شَهِدَ اللَّـهُ Shahidal-lahu (3:18) and قُلِ اللَّـهُمَّ مَالِكَ الْمُلْكِ : Qulil-lahumma malik al-mulki upto بِغَيْرِ حِسَابٍ bighairi hisab (3:26, 27), Allah Almighty will forgive all his sins, admit him to Paradise and take care of seventy (i.e. plenty) of his needs, the simplest of these being his forgiveness.' (Ruh a1-Ma` ani with reference to Dailami)