Witnesses do all of this because they are convinced that they have found the truth.
إِنَّ كُلَّ هٰذِهِ ٱلْجُهُودِ ٱلَّتِي يَبْذُلُهَا شَعْبُ يَهْوَهَ نَابِعَةٌ مِنْ قَنَاعَتِهِمِ ٱلرَّاسِخَةِ بِأَنَّهُمْ وَجَدُوا ٱلْحَقَّ .‏