A more important point is that many do not believe because there is no evidence. This is a logical fallacy (argument to ignorance type). A lack of evidence never disproves a thesis. Of course, it doesn't prove it, either.
So many people are afraid to say simply, "I don't know." For some, this is an ego thing. They have to pretend that they know, because the unknown is so uncomfortable to ego.
For someone who thinks that they are only their body and are comfortable with that, the question is entirely moot. But claims made by disbelievers are worthless. Again, their arguments are based on a logical fallacy. Saying, "I don't know," would be far more productive.
As others have so accurately put, invisibility is not a valid basis for non-existence. Wind, gravity, time, space -- all of these, and more, are invisible.
Some people are not ready to sense spirit. That's okay. Move on to those who are ready. Those who are ready for it can feel it. Disbelievers may think this is delusion, but that's only their own blindness.
Exploring the non-physical (spiritual) has led to many breakthroughs in my life. These include viewing the environment with 20/20 clarity without human eyes, and miracles of the extraordinary (cause-and-effect coincidence) kind. This state has also led intellectual discoveries only later confirmed by research. It's as if I could "see" the knowledge first, without having to "learn" it. The learning (confirmation) came later. More importantly, this state has helped achieve greater understanding of basic principles. Like the time I was studying electronic engineering and realized that I was surrounded by trillions of tank (LC) circuits. This was helpful in developing my 3D astronomy software, Stars in the Neighborhood and in writing my two novels. It also helped in my discovery of a biblical timeline compatible with those of science and the discovery of the Kabbalists' "Tree of Life" matrix embedded in two chapters of Genesis, as well as the identity, through science, of the Genesis 6 "daughters" responsible for the event we call "The Flood."