Chaplain is a hidden job class in Tree of Savior, and requires one to finish a hidden quest make it available on the class advancement screen. Thankfully, unlocking and completing the Chaplain class quest is quite simple, and I will show you how.
You need to be Priest Rank 3 in order to be able to change to Chaplain, but you can still unlock the class first.
Talk to a guard in Orsha until he tells you that he recently met the Chaplain Master.
This is the second guard standing beside the Lord of Orsha. It will take multiple clicks on him for him to eventually reveal the dialog about the Chaplain Master.
Go to Klaipedia, and exit to East Siaulai Woods.
Talk to the guard on the left of the entrance to Klaipedia, and he will tell you to talk to the Priest Master in Klaipeda.
Talk to the Priest Master at Klaipedia.
He will tell you that the Chaplain has joined forces with Commander Julian’s forces. That is in fact, in Dvasia Peak.
Go to the Dvasia Peak way point.
You will find the Chaplain Master standing to the right of the Statue of Goddess Vakarine. Talk to him and you will be able to advance to Chaplain from there on.
Finish the Chaplain class quest which is just killing a few mobs on the map.