What you believe? Or the way you believe it?
Fundamentalism has less to do with the contents of belief than the way the believer holds those beliefs.
For example, fundamentalism is strongly associated with dogmatism — beliefs considered unassailable and held with fervent, unjustified certainty[…].
[F]undamentalism has a strong “us versus them” dynamic, […] authoritarianism and ethnocentrism (e.g. Positive correlations have been observed between fundamentalism, anti-homosexual sentiment, and racist attitudes.)
—”Experimental Theology: The Psychology of Christianity”