"Then He will judge between us in truth," meaning: He will adjudicate. "And He is the All-Judging (al-Fattah)," the Judge, "the All-Knowing (al-'Alim)" of His creation.
﴿ثمَّ يفتح بَيْننَا بِالْحَقِّ﴾ أَي: يقْضِي ﴿وَهُوَ الفتاح﴾ القَاضِي ﴿الْعَلِيم﴾ بخلقه.