I do not believe in anything, so I do not believe that aliens exist.
I also do not believe that they don't.
Mathematically speaking, they most likely do exist, especially if you forget about limiting the type of life to "humanoid" and accept that alien life may not be limited to what we know is the definition of life. There may be four or five dimensional beings, beings that stretch out across the cosmos, beings that are not carbon based, or sentient life that is microscopic.