The words in أَنتُمْ تَشْهَدُونَ (you are yourselves a witness - 70) and in أَنتُمْ تَعْلَمُونَ (you know ( 71) should not be taken to mean that disbelief will become permissible for them if they do not attest to the truth or have no knowledge about it. This is because kufr (disbelief) is intrinsically an abominable thing to practice. This is prohibited under all conditions. However, the mention of kufr (disbelief) after knowledge and attestation is to increase the impact of reproachment.