The Quran

وَالَّذينَ آتَيناهُمُ الكِتابَ يَفرَحونَ بِما أُنزِلَ إِلَيكَ ۖ وَمِنَ الأَحزابِ مَن يُنكِرُ بَعضَهُ ۚ قُل إِنَّما أُمِرتُ أَن أَعبُدَ اللَّهَ وَلا أُشرِكَ بِهِ ۚ إِلَيهِ أَدعو وَإِلَيهِ مَآبِ

Muhammad Asad

Hence, they unto whom We have vouchsafed this revelation¹ rejoice at all that has been bestowed upon thee [O Prophet] from on high;² but among the followers of other creeds there are such as deny the validity of some of it.³ Say [unto them, O Prophet]: "I have only been bidden to worship God, and not to ascribe divine powers to aught beside Him:⁴ unto Him do I call [all mankind], and He is my goal!"

Sc., "and who believe in it".
I.e., because it offers them guidance in this world and holds out to them the promise of ultimate happiness in the life to come.
I.e., while admitting that the Qur'an contains much that coincides with the spiritual concepts taught by their own religions. The designation ahzab (lit., "parties" or "sects", sing. hizb) connotes here the followers of other religions or creeds (Tabari and Razi).
The particle "only" (innama) at the beginning of this sentence "clearly shows that there is [in Islam] no obligation, no ordinance and no prohibition that is not connected with this [principle]" (Razi).