"The truth" refers to Muhammad (peace and blessings of Allah be upon him), "while you know" means: you find him mentioned in your scriptures.
﴿الْحق﴾ يَعْنِي: مُحَمَّدا ﷺ ﴿وَأَنْتُم تعلمُونَ﴾ أَي: تجدونه مَكْتُوبًا عنْدكُمْ
