Firstly, you must select the Jew class at the beginning of the game to unlock this achievement. If you want to play as a different class, this achievement can be gotten almost right away, so no worries. Once you finish the tutorial and can leave Kupa Keep, pull up your map and head to City Hall. Talk to Father Maxi to start his side quest. Go to the Church and find Jesus, who is hiding behind a pew. Go back to Maxi to continue the quest. Return to the church one more time and this time Jesus is hiding behind the altar. The achievement will unlock, as well as giving you Jesus as a friend, and granting you his Summon ability.
For this, just when the game starts and Cartman asks you which class are you, answer Jew. Once the first part finishes and you're free, go find father Maxi and you'll have to finish this mission to get the achievement.At the beggining of the video I speak in Spanish but it's just and introduction and I don't say anything relevant.