Learn English – Word for a person who has no opinion about whether god exists

Perhaps scientific

conforming with the principles or methods used in science: a scientific approach

The scientific method is defined as

principles and procedures for the systematic pursuit of knowledge involving the recognition and formulation of a problem, the collection of data through observation and experiment, and the formulation and testing of hypotheses

Scientists (and others practicing the scientific method, in work or their everyday lives) routinely have viewpoints, opinions and hypotheses about issues, but are ready to modify those positions based upon new evidence.

And, in anticipation of any objection that the initial example given was in the religious context, numerous scientists believe that their scientific approach is not incompatible with their religious beliefs, and many argue that their scientific approach demonstrates the validity of their religious positions.

Albert Einstein provides an example of the view that science and religion are compatible.

Spiritual would be the best way to describe yourself.

There is a movement to try and differentiate the terms spirituality vs religion (Sources are numerous, here's one)

Also, there is a belief system being adopted called Spiritual but not religious (SBNR) that is used to self-identify a life-stance of spirituality that rejects traditional organized religion as the sole or most valuable means of furthering spiritual growth.

However, there are several beliefs that you could identify with.

1. Thanatism (as pointed out by another user) where the basis of belief is life comes before death, not after it (no afterlife). The term Thanatism comes from thanato, which means death. After an individual dies, their soul becomes one with nature. Thanatism is type of spiritual belief and not a religion.

   I'm a thanatist. I live life to its fullest.

2. Esotericism or esoteric, which means that you believe in something that few others do. The term translate to 'inner-circle' and is used to define a small group of individuals that believe in the same thing.

   I don't subscribe to any mainstream religious thoughts. My beliefs are esoteric.

3. Theosphical whereby you use your own beliefs to justify certain facts for your spiritual or religious benefit. One example of this is Christians trying to justify dinosaurs existing during the early history of the world.

   I believe in everything the Bible says except about heaven and an afterlife.

4. Mysticism is "a constellation of distinctive practices, discourses, texts, institutions, traditions, and experiences aimed at human transformation, variously defined in different traditions." Due to its diverse interpretation, you can incorporate a believe (or lack thereof) about the afterlife.

   I don't believe in an afterlife for the soul because I'm a mystic.