All religions are true, according to Sikhism.
In Sikhism, it is believed that all religions have a core of truth in them and all religions can lead you to the same goal -- eternal enlightenment (sometimes called nirvana, heaven, etc.). It's like there are different hiking groups all trying to reach the top of the mountain. Some groups follow the trail with the blue marker, others follow the trail with the red marker, another follows the green marker, etc. Some take longer to reach the tops than others (Sikhs think they're on the ski lift 😉 as we think we have the most direct path to the Great Spirit).
The One Spirit shows itself to different groups of people in different ways. People developed around different weather patterns, different physical environments, etc. and therefore developed a variety of cultures. The Spirit can all be found in all religions and spiritualities.