Hey, since this is posted and my guide got roll backed due to some technical difficulties in the forum, I'll just paste mine here to reduce redundancy.
It's more or less the same "How to use Pixies guide" with more steps.
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Good Day Valiants!
Christmas is around the corner and Pixies have appeared!
In this guide, I will show you how simple it is to change faiths with Pixies. 
To look at rerolling hero faith normally without Pixies, please look at Satsuki's Guide on The different Faiths available for all heroes or Hayzen's Faith Guide w/Charts.
Satsuki:
To strengthen your Squad, first you have to acknowledge your Heroes' Faith!
You can find the small button near your heroes' name on the right side (visual guide below). Each hero will have his or her own Faith, affecting stats growth as they level up.
There are 6 different Faiths, as seen above and each new hero will come with one of the Faiths at random. The grade (Strong, Weak, etc) of how each Faith affect stats growth will never change. For example, the Ares faith will always have a "Strong" grade for ATK, thus it may be more suitable for the damage-dealing classes.
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So you're planning to get a Pixie or you have already got one and wondering about "How do I change the faith with Pixies?" or "Is changing the faith with Pixies the same as rerolling faith?".
Currently, from what I know, Pixies can be obtained randomly from Christmas Tickets or 100% chance to choose a Pixie of your choice in the Christmas Bundle.
Below are some simple steps to taken to clarify your thoughts and change your chosen hero faith with a Pixie!
Step 1:
At town, click the
hero icon on the bottom
left-side to display a menu. Then proceed to click on the "Hero" tab in the middle of the popped up menu.
Step 2:
Look for a hero that has undesirable Faith that you want to change. In my case, I have chosen Victora shown below.
Step 3:
Click the button on the heroes' name
right-side to check what Faith the particular hero has. If you wish to change the hero Faith with a Pixie, click on the green button "Reroll Faith".
Step 4:
To change the Faith of your hero with a Pixie, you must have a 3-star Pixie.
Example: If you want to change the faith of Victoria to Kratos, you must
sacrifice a "Pixie of Kratos". (Pixies can be used to change any hero Faith, 1 - 5 stars, except for Valiants)
Make sure you have
sufficient gold and your Pixie is not
locked or in any
squad.
Step 5:
A confirmation screen will pop up asking "Are you sure you want to change the faith of the selected unit?".
Confirm your decision. Then the system will change your hero's Faith to the name of the Pixie Faith!
Do Note:
If you are not sure if the sacrifice is a Pixie, check the
Gallery tab under Hero and look for
Pixies of Faith under
Special Tab.
When sacrificing a Pixie, the hero Faith will only change to the name that the Pixie Faith has.
Example: "Pixie of Kratos" will always change hero's Faith to Kratos.
Upon attempting to use a Pixie, it will always say
"100% chance Faith change to [Name of Pixie Faith]". Example shown below.
Thanks for reading my guide! I hope this clarify your thoughts about Pixies.
Be Valiant, Play Fair,
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