In the original language, that phrase means “to be assured of ” or“ to be convinced and certain of the truth of something. ”
وَهٰذَا يَعْنِي أَنَّهُ كَانَ وَاثِقًا تَمَامًا أَنَّ هٰذَا هُوَ ٱلْحَقُّ .‏