﴿مُصَدِّقًا لِّمَا مَعَكُمْ﴾ “Affirming what is with you”: referring to the Torah. And what is intended here is that they believe in the Noble Qur’an that came to affirm all the previous divine scriptures.
And that Sayyiduna Isa u gave the glad tidings of this last scripture, as Allah U said in the Holy Qur’an:
﴿وَإِذْ قَالَ عِيسَى ابْنُ مَرْيَمَ يَا بَنِي إِسْرَائِيلَ إِنِّي رَسُولُ اللَّهِ إِلَيْكُم مُّصَدِّقًا لِّمَا بَيْنَ يَدَيَّ مِنَ التَّوْرَاةِ وَمُبَشِّرًا بِرَسُولٍ يَأْتِي مِن بَعْدِي اسْمُهُ أَحْمَدُ ۖ فَلَمَّا جَاءَهُم بِالْبَيِّنَاتِ قَالُوا هَٰذَا سِحْرٌ مُّبِينٌ﴾
“And [mention] when Isa, the son of Maryam, said, “O children of Israel, indeed I am the messenger of Allah to you confirming what came before me of the Torah and bringing glad tidings of a messenger to come after me, whose name is Ahmad.” But when he came to them with clear evidences, they said, ‘This is obvious magic.’” [Surah As-Saf: 6].