Renowned for his ability of boosting the morale of members of the Great Trek during times of hardship, he would often inspire troops through his recital of the Lord's Prayer during conflict. His skills of fortifying the troops were best displayed at the Battle of Blood River, where he spoke the famous vow to God on the battlefield, and held a thanksgiving service after the decisive victory over the Zulus. Cilliers' vow is celebrated as a national holiday in South Africa (Day of the Vow) on 16 December.