Aikido seems more like a medium, just as oil painting is a medium for Art. People can express themselves through Aikido just like oil painters can express themselves through their paintings. An oil painter can be a member of different schools of art, just as an Aikidoist can be a member of different schools or religions.
An artist might want to use a different medium, like watercolors, dance, theater or sculpture. A martial artist might want to train in TKD, Karate or Judo.
What matters is the individual expression. Maybe that's why Aikido is not a religion but a martial art.
Onisaburo Deguchi used to say that art is the mother of religion. I always had difficulties to understand this doctrine. Had it been "religion is the mother of art" I would have agreed. Religious faith is often expressed in art - and so did O-Sensei with Aikido