I'm not sure if Plato or Aristotle have any works that are exclusively about religion like, say, David Hume's Dialogues Concerning Natural Religion. If they do, that'd be easier; but, I imagine their religious views pop up throughout various of texts. Which of their texts would I be able to read to understand their views on religion?
1 The most obvious Platonic dialogue is the 'Euthyphro' which discusses the question whether the holy [or right] is loved by the gods because it is holy or holy because it is loved by the gods. (Eu., 10d 6-7.)
2 In the 'Republic' Plato purges education of myths about the gods which represent them as arbitrary, untruthful, mischievous, cruel. (Rep., 377e ff.) Whether the Form of the Good (Rep., 532ab, 534bc) is divine or godlike is controversial but Plato's remark that 'the truth of the matter is, after all, known only to god' (Rep.VII. 517b : Lee tr.) suggests a distinction between the Form of the Good and god.
3 The 'Timaeus' is also relevant. While discussing the origin of the cosmos, Plato underlines that God brought order into the prevailing irregularity and disorder : 'out of disorder he brought order, considering that this was in every way better' (Tim., 30a 5-6 : Jowett tr.).
4 In the 'Laws', his last major work, Plato maintains that God or gods exist and pronounces them 'the measure of all things'. (Laws, IV.716 c4 : AE Taylor tr.) They are the causes of all things; their knowledge is perfect; they are perfectly good and may be omnipotent. There is also talk of the enforcement of religion in the dialogue and Bk X addresses the issue of atheism.
5 The immortality of the soul (psuche) is discussed in the 'Phaedo' (Phaedo, 106a-e but that's only one of several arguments concerning immortality in the dialogue.) The Myth of Er in the Republic also concerns the fate of souls in the afterlife. (Rep., 608c ff.). Diotima's speech in the 'Symposium' touches on the topic of immortality. (Symp., 208a-b, 212a.)
6 Aristotle identifies God with the 'prime mover'. See 'Metaphysics', Λ.7 : XII.7; also 'Physics', VII & VIII.) God does not intervene in the world. God is 'pure thought thinking about pure thought'; God's existence and activity involve solely self-contemplation : perfection thinking about perfection. Movement, change, and development in the world are all due in ways not possible to explain in this short space to the attempt of everything in its own fashion to imitate the divine perfection.
7 In 'de Anima' (III.5) the relation of the 'active intellect' to the prime mover of 'Metaphysics', Λ.7, is contentious.
8 In many other places references to God occur in Aristotle's writings : 'Topics' (e.g. II.2, III.1), 'Prior Analytics' (e.g. I.36), 'Nicomachean Ethics' (e.g. I.6). There is unfortunately no room here for a full list.