I recall the arguments that I've had with some of my friends who would propose that there are many atheistic/agnostic folks who are good, decent people, and there are so many religious people who are such arseholes. While I acknowledge this was difficult to disagree from by the fruits you shall know position, I've always believed that there is a deeper way to consider this. As non-believers can certainly be good and believers can definitely be jerks, but it appears only to a point.
One of the most sophisticated takes on morality came from Alasdair MacIntyre's classic book After Virtue. I will warn you it's not an easy read, but the points are good. MacIntyre does a great job positing how the modern project via the Western Enlightenment thinkers (Kant, Bentham, Mills) did a disservice to morality by freeing it from the Christian/Aristotelian teleology. Then along came Nietzsche who had a field day dismantling all this utilitarian/deontological nonsense that had little foundation for support. He then opened the door for the existentialists and the moral relativists which allowed for the fragmented, individualistic world of morality we inhabit these days.
MacIntyre posits that morality only has a rational ground when it is based on teleology (an overarching narrative), because it is only then when one can say whether or not something is good or bad in relation to achieving that shared good. The telos gives unity to the diversity of narratives, and this whole of narratives offers clarity for a moral vision to ensue.
The Christian/Aristotelian teleology aims the primary goal as sanctification/divinization/theosis for each person, and love becomes the desire to will the good of the other in that story we all play our part in. That love can only come from a source greater than ourselves, otherwise it will always be conditional.
William Wildblood says, “True love [and the morality that ensues] only comes from going beyond yourself, or even other people, to a higher, transcendental truth and that is God. In effect these people are counselling the horizontal path without acknowledging the vertical but the horizontal without the vertical keeps you firmly in this world with its falsehoods and illusions, and you will never find height or depth.” Hence, the secular humanist will always be limited by his potential for the good.
And as for all those religious people who suck. Wildblood notes that, “Firstly, the process is a long one and depends on the base level at which you start. And secondly, many people only believe in their heads but it is the heart not the mind that matters. Theoretical belief is of little use if the heart and the imagination are not involved. That is because only belief in the heart and the imagination affects the whole person.”
I would also add that your deeper beliefs and intellectual understanding do matter. You may belong to a religion that has a poor teleology (see Islam) or one that does not have much of a teleology (see Buddhism). As such, it is imperative to believe wisely, open your heart to Divinity, and be good Johnny.