Halogen-bond (XB) catalyzed reactions could also be catalyzed by Brønsted acids, which may complicate the picture of the true activation pathway for these reactions. Herein, we report the first density functional theory study of the mechanistic pathways for the uncatalyzed, iodoimidazolinium halogen bond catalyzed, and a competitive Brønsted acid-catalyzed reduction of quinoline by Hantzsch ester. The uncatalyzed reaction was found to proceed via stepwise pathways. In the lowest energy pathway, proton transfer from Hantzsch ester, a weak Brønsted acid, to quinoline prior to hydride reduction was identified as the key to the lowered energy barriers compared to other reaction pathways. The same reaction steps are involved in the XB-catalyzed pathway, but with substantially lowered reaction barriers, particularly for the hydride-transfer steps. In contrast to the general belief that halogen bond catalysts bind to the electrophile quinoline and activate it by lowering its LUMO energy, we discovered that it is preferable to lower the LUMO energy of quinoline through protonation by Hantzsch ester as a Brønsted acid and stabilize the conjugate anion of Hantzsch ester via halogen bond. Finally, our calculations reveal that the iodoimidazolinium type of catalyst is prone to reduction by Hantzsch ester, generating a Brønsted acid as product. The Brønsted acid catalyzed pathway was calculated to be competitive with the halogen bond catalyzed pathway. Our theoretical findings highlight the need to be cautious when applying iodoimidazolinium catalysts in organocatalysis, and we hope it will aid the design of new halogen bond catalysts that could avoid undesirable Brønsted acid catalysis.