This should hopefully save you from having to wade through the 20 page summary in one of the older Edexcel textbooks
In an important survey article on developments in philosophical theology and the philosophy of religion, Merold Westphal notes how different philosophers have tried to distinguish the ‘kernel’ or essence of religion from the ‘husk’ – its less essential elements. This is an important distinction because if we are going to critique religious beliefs in any way, there has to be agreement on what those core beliefs are. What Westphal’s article demonstrates is a lack of consensus about this.
For example, for Kant, this kernel cannot include beliefs about God and arguments to prove the existence of God. Although Westphal doesn’t go into detail, this is because (without getting too technical), Kant’s philosophy is fundamentally based on a distinction between things as they are in themselves and things as they appear to us, a secondhand reality that has been first of all processed by the mind. The unknowable nature of true reality is something that God belongs to. So for Kant, the best we can do is to focus on a rationally derived, duty-based ethic. This is his ‘kernel’, and so a critique of religious beliefs would need to start with his ethical system and his claim that ‘morality’ nevertheless ‘leads inevitably to religion’, a position supported by his claim that there are good practical reasons for believing that God exists, even though there is no absolute proof.
But for Schleiermacher [NOTE: take care with the spelling of his name!], even morality is part of the ‘husk’ of religion, along with metaphysics. The essence or kernel of religion is not actually a belief at all. It is a ‘feeling’, an awareness of ‘the Eternal’. Still other philosophers and theologians have tried to identify the rational kernel of religion, in order to separate it from an irrational husk. Examples of the rational kernel in this respect involve conceptions of God as the creator, or as the guarantor of the moral law (the line taken by Kant) as opposed to more rationally dubious claims about miracles and other supernatural elements of religion (e.g. a belief in angels). That this kind of project was attempted perhaps indicates that the simple relegation of all religious beliefs to the realm of the irrational by authors like Richard Dawkins caricatures what religious belief has historically been taken to consist of.