[1] Islam is the chosen religion to which all the Prophets called in every age and which they all taught. And now, the complete form of this Islam is that which the Final Prophet Muhammad (ﷺ) presented to the world. According to it, one must believe in Tawhid, Risalah and the Hereafter exactly as the Noble Prophet (ﷺ) instructed. Now, Islam is not established, nor is salvation in the Hereafter attained, merely by considering Allah to be One or by doing some good deeds. Rather, the requirement of faith and Islam is to regard Allah as One and worship Him alone, to believe in all the Prophets including Muhammad (ﷺ), and also to accept that no Prophet will come after the Noble Prophet (ﷺ).
Along with this faith, one must hold to the beliefs and practise the deeds described in the Quran and Hadith. Now, no religion other than Islam will be accepted by Allah.
Allah says: "And whoever seeks a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers."
(Surah Al 'Imran 3:85)
The Prophethood and Messengership of the Noble Prophet (ﷺ) is for all of humanity.
[قُلْ يَا أَيُّهَا النَّاسُ إِنِّي رَسُولُ اللهِ إِلَيْكُمْ جَمِيعًا]
"Say: O mankind! I am the Messenger of Allah to you all."
(Surah Al-A'raf 7:158)
[تَبَارَكَ الَّذِي نَزَّلَ الْفُرْقَانَ عَلَى عَبْدِهِ لِيَكُونَ لِلْعَالَمِينَ نَذِيرًا]
"Blessed is He who sent down the Criterion (Al-Furqan) upon His servant, so that he may be a warner to all the worlds."
(Surah Al-Furqan 25:1)
In a hadith, the Messenger (ﷺ) said: "By the One in whose Hand is my soul! Whoever among the Jews and Christians dies without believing in me will be among the people of Hell." (Sahih Muslim) He also said, "I have been sent to the red and the black." (That is, I have been sent as a Prophet to all people.) And for this reason he sent letters to all the kings of his time, inviting them to accept Islam.
(Bukhari, Muslim, Ibn Kathir)
[2] 'Disagreement' refers to the mutual disputes that the followers of one and the same religion stirred up among themselves — such as the disputes, divisions and factionalism among the Jews, and likewise the conflicts and divisions among the Christians. It also refers to the disagreements between the People of the Book themselves, because of which the Jews and the Christians would say to one another, "You have no foundation to stand on." The dispute over the Prophethood of Muhammad (ﷺ) and of 'Isa (peace be upon him) is also included. None of these disagreements were based on evidence; they arose solely from envy, malice and enmity.
That is, they knew and recognized the truth; yet despite this, chasing whimsical worldly interests, they clung to false statements and tried to present them as the religion — so that their prestige would stay high and their standing among the people would remain secure. Regrettably, nowadays a large number of Muslim scholars are following the very same misguided path, driven by exactly these kinds of vile motives. May Allah guide them and us. Ameen.
[3] Here, 'signs' refers to those signs which prove that Islam is the religion chosen by Allah.