Webster: Intelligence- the ability to acquire and apply knowledge and skills. So anybody and everybody can be and usually is intelligant in some form, so for me the difference of being religious or being non-religious is truly apples and oranges, neither is 'better' or 'smarter' but different.

The following is from a study published Frontiers in Psychol., 19 December 2017

This was a very big study. Note the last sentence.

"It is well established that religiosity correlates inversely with intelligence. A prominent hypothesis states that this correlation reflects behavioral biases toward intuitive problem solving, which causes errors when intuition conflicts with reasoning. We tested predictions of this hypothesis by analyzing data from two large-scale Internet-cohort studies (combined N = 63,235). We report that atheists surpass religious individuals in terms of reasoning but not working-memory performance. The religiosity effect is robust across sociodemographic factors including age, education and country of origin. It varies significantly across religions and this co-occurs with substantial cross-group differences in religious dogmatism. Critically, the religiosity effect is strongest for tasks that explicitly manipulate conflict; more specifically, atheists outperform the most dogmatic religious group by a substantial margin (0.6 standard deviations) during a color-word conflict task but not during a challenging matrix-reasoning task. These results support the hypothesis that behavioral biases rather than impaired general intelligence underlie the religiosity effect."

Intellect doesn't necessarily translate to religious belief.