Must put on (δε ενδυσασθα). Aorist (ingressive) middle infinitive, put on as a garment.

Immortality (αθανασιαν). Old word from αθανατος, undying, and that from α privative and θνησκω, to die. In N.T. only here and 1. Timothy 6:16 where God is described as having immortality.