It was in his childhood that Cilliers's religious values and aptitude for spiritual leadership were first developed, as he was said to have had a personal encounter with God at age 10.

In 1835, a drought hit the local area, and this led to Cilliers joining the Great Trek with his wife and six children in 1836, as he was convinced that it was God's will that he should go.

Cilliers was made the unofficial pastor of the Great Trek, and he would hold services on a daily basis and give Sunday Communion.

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.

[5] The official journal of the emigrant Boers' punitive expedition against Dingane and his Zulus was published in De Zuid-Afrikaan of Friday 14 June 1839.

The three morning services were led by three men: De heeren Cilliers, Landman en Joubert, waren opgeruimd in het gemoed, om zulks te horen; zy onderhielden hunne gemeentens hierover en verkregen hunne algemene toestemming.

which in English literally means "Sunday, 9 December 1838 The next day, the 9th, everything was still quiet, and we stayed here to celebrate the Sabbath; while also the previous Saturday evening, in the tent of the Commander-in-Chief, was spent with the singing of some appropriate songs, and a powerful prayer, performed by Mr. Cilliers.

Making that vow on Sunday morning, before the commencement of religion, the Chief Commander sent those (Cilliers, Landman and Joubert – see below) who would practice the religion to come to Malkander, and asked them, to speak to the congregation, that all of them mightily persevere in spirit and in truth, praying to God for His help and assistance in striking the enemy; that he wanted to make a vow to the Almighty, (if all would like to), – "so that the Lord may give us the victory, to establish a House of Remembrance of his Great Name, where it will please Him," – and that they also had to plead, the help and assistance of God, to be able to fulfill this vow surely, and that we will record the day of victory, in a book, to make it known, even to our last descendants, that The glory of God may be celebrated.

The three morning services were led by three men: The Messrs. Cilliers, Landman and Joubert, were delighted to hear such; they maintained their congregations on this and obtained their general consent.

In 1843, after the British annexation of Natal, Cilliers withdrew from political life, maintaining an important role as an elder of the church.

He died on 4 October 1871, and was buried in the Cilliers family cemetery, in a grave marked by a simple tombstone and later a monument.

Although he is most famously remembered for speaking the Vow at the Battle of the Blood River, he was, in addition, an important progenitor of the early Dutch Reformed Church in South Africa.

He is remembered as being a constantly determined, principled, and often selfless individual, shown through his ability to endure hardships to serve others in need.

Gerdener wrote a biography of Sarel Cilliers in 1919, which includes details of various events in his life and a reconstruction of the Vow.