﴿ وَإِذَا يُتْلَىٰ عَلَيْهِمْ قَالُوٓا۟ ءَامَنَّا بِهِۦٓ إِنَّهُ ٱلْحَقُّ مِن رَّبِّنَآ إِنَّا كُنَّا مِن قَبْلِهِۦ مُسْلِمِينَ ﴾

“and whenever it is [clearly] conveyed unto them, they [are bound to] profess, “We have come to believe in it, for, behold, it is the truth from our Sustainer - and, verily, even before this have we surrendered ourselves unto Him!””

Please check ayah 28:55 for complete tafsir.