Angels are massively powerful entities that inhabit the astral plane, separated from our world through a metaphysical barrier. They can only enter our world by way of a summoning spell, a form of ritualized magic which relies on the power of nature. Unlike other forms of magic which depend on the mana level of the user, ritualized magic uses the unlimited mana of nature itself, allowing for the most powerful of spells. These spells are slow and complicated, requiring various items and multiple steps that must be carried out in a certain way.
The setup of a spell requires 7 female witches and one male warlock. The individuals enter a prepared ritual circle, with the 7 standing on the edges of it and the warlock inhabiting the center. The participants begin to sing and chant, forming a ritualized dance to summon the creature. Through this method, an angel can cross over into our realm and be made to do a number of things, such as answer questions, attack opposing forces, etc. The stronger the entity , the more ritual circles with the same setup are needed. Multiple circles must be connected with each other and work in tandem to summon more powerful entities.
However, even after the entity has crossed over, the summoning spell must continue. As the spell depends on the unlimited magic of nature rather than the individuals, the creature should be able to maintain its presence after being summoned. However, if the ritual doesn't remain active, the entity would lose its connection to our world and fade back to the astral plane. How can this be the case?