Rentzing studied law and philosophy at the Free University of Berlin from 1987. It was not until 1989 that he additionally devoted himself to Protestant theology in Berlin. Rentzing moved to the Johann Wolfgang Goethe University in Frankfurt am Main in 1992 and then studied at the Lutheran Theological College Oberursel of the Independent Evangelical Lutheran Church.  From 1997 to 1999 Carsten Rentzing completed the preparatory period with congregational internship in the parishes of Oelsnitz/Vogtland and Zwota in Saxony. In 1999 he passed the Second Theological Examination in Dresden and was ordained as a pastor in Annaberg-Buchholz, in the Ore Mountains, where he also took up his first pastorate, but at the same time completed his doctorate at the University of Leipzig until 2003. In 2004, Rentzing co-founded the Martin Luther League in Saxony and in 2016 became a member of the board of trustees of the evangelism association ProChrist. From 2015 until his resignation in 2019, he was the regional bishop of the Evangelical Lutheran Church of Saxony (EvLKS).