Determining Your Level
To determine a deck's shared level, you must look at the highest requirement per attribute of cards in your deck. For example, If you include the card Healing Hammers as the first card in your deck, your deck would be considered to be "Level 4 Faith". This is because Healing Hammers has an attribute requirement of four Faith. From this point forward, any other cards you add to your deck that have a requirement of four or less Faith will have no impact on your deck's level. You may add Guided Strike and Arms of the Goddess, your deck is still "Level 4".
But what if you add a card from another attribute? What if you really want Spellbreaker in your paladin deck?
Spellbreaker has an attribute requirement of three aggression. Because Spellbreaker requires an attribute that is not apart of your deck yet, it will count toward your level. At this point, your deck would be considered to be "Level 4 Faith, Level 3 Aggression", also known as a "Level 7".
At this point, if your goal was to make a level 7 deck, you would be at max level. You could only add actions to your deck that had a faith requirement of four or less, and an aggression requirement of three or less. You could add some Critical Strikes and an Enrage or two just fine, but if you decide that you really need that Thy Will Be Done then you are going to need to remove all actions in your deck that require more than one aggression.