ٱتَّخَذُوٓا۟ أَيْمَـٰنَهُمْ جُنَّةً فَصَدُّوا۟ عَن سَبِيلِ ٱللَّهِ فَلَهُمْ عَذَابٌ مُّهِينٌ Qur’an Al-Mujadilah (58:16)Ittakha th oo aym a nahum junnatan fa s addoo AAan sabeeli All a hi falahum AAa tha bun muheen un
I.e., by sowing doubts in other people's hearts.
A false man, by swearing that he is true, makes his falsehood all the more heinous. He stands in the way of other people accepting Truth. He gives a handle to the cynics and the sceptics.