If God can affect humans, then there is a context in which God and humans coexist.
If this context exists, then humans should be able to access God.
If God can be accessed, then he can be observed and measured, thus proving the existence of God.
The fact that no theist has embarked on a successful quest to find this shared context and to access God for observation and measurement admits to one or more possible explanations. They may be too lazy. They may be afraid of what they'll find (which admits to a number of possible conclusions itself). The may not be capable of doing so. It may not exist. There are probably others that I've missed, but all of this leads us to a conclusion that God is not very probable, given that the people having the vested interest in proving the existence of God haven't done so.
That also leads to another possible conclusion - that they have a vested interest in not proving the existence of God, which allows them to define God for their own mysterious purposes.