To get the Paladin stronghold, choose Paladin as the class for your main character. Either accept the quest from the three dryads in the prologue, or go to the Slums district in Athkatla and speak with Lord Jierdan inside the Copper Coronet. The dryads will give you the quest "Helping Irenicus' Captive Dryads", whilst Lord Jierdan will give you the quest "Journey to the Windspear Hills". You can choose to take on either or both, then travel to Windspear Hills. Kill the attackers and Garren Windspear will initiate a dialogue with you. Join him to his hut, and he'll let you stay while he goes on a trip. Fight off the bandits, whom teleports Garren's child away. As Garren returns, he'll give you the "Rescue Garren's child from Firkraag" quest. Go to the Northeast and enter Firkraag's Lair. Complete the quest, making sure to rescue Garren's child and not making a deal with Firkraag. Return to Garren to complete the quest. Travel to the Temple district in Athkatla, and enter the Order of the Most Radiant Heart. Speak with Prelate Wesselan in his office here and accept becoming a member of the order. The achievement will then unlock.
Note 1: This can be done immediately after the prologue and speaking with Gaelan Bayle.
Note 2: Apparently, forcing a confrontation with Firkraag can make it impossible to get the stronghold. I had no trouble with that however, killing Firkraag before rescuing Garren's child, so it might be a bug that's been fixed. But it can be a good idea to make a manual save before speaking with Firkraag, just in case.