Technically speaking, this poem doesn't actually have a title. But lucky for you, we'll talk about it anyways.
Like many of Cummings's poems, this piece is simply known by its first line. It's not printed with a title, so the first line becomes the default title, and there' you have it, folks: "if everything happens that can't be done."
As luck would have it, this first line/title comes in handy when we're trying to understand this poem. The piece starts off by proposing a hypothetical situation, in which everything happens that can't be done. We're given the first half of an "if… then" statement. When you read every line keeping the first one in mind, it can help you make sense of some of the paradoxes and contradictions in the poem. After all, if the impossible can happen, well, then the impossible can happen.
Plus, it's important to note that the first line doesn't just come right out and say, "Look, you can do the impossible!" Instead, it tells us that everything is happening that can't be done. People aren't doing these things that can't be done. Nope, those things are just happening. There's some mysterious force at work here, and we're guessing that it might be this whole "one" concept, and we're guessing that it has something to do with love.