In philosophy, there is a distinction between things that must be (they're called necessary) and things that may or may not be (they're called contingent). For instance, if we use the standard definition of a square, then every square you will ever encounter will necessarily have four sides. In contrast, not every square you encounter will be blue or tiny. Those are contingent properties of a square. A square may be blue, or it may not be. A square may be tiny, or it may not be.
Now, would you say that our universe's existence is necessary or contingent? If its existence is necessary, it would mean that there is no possible way for our universe not to exist. If its existence is contingent, then our universe either could exist, or it could not.
I happen to think it's contingent. But if our universe is contingent, then it must be contingent upon something that is necessary. Regardless of whether you view that necessary thing as God or something else, if you agree that the universe is contingent, then the contingency doesn't stop at its creation; its continued existence is also contingent.
So, beyond needing a "first cause" to explain its existence (as with a picture on a painted canvas), our universe requires a "sustaining cause" (as with a picture on a TV set).