Worship. Marcus Aurelius 3.13

Like many throughout history, Marcus Aurelius does not approach religion strictly in terms of belief. As a practical Roman and pagan, he recognizes straightforwardly the existence of natural forces that elude human control: religion for him is a series of rituals we engage to show proper respect for our limits in handling ourselves around these forces. What we call them or believe regarding them at any particular moment is never as significant as cultivating regular methods of showing respect that can help us and others remain calm around them. We can appreciate a position like this better if we compare it with our own attitude toward trial by jury: one does not disprove, or seek to disprove, the efficacy of trial by jury by noticing that its justice occasionally miscarries. The ritual has power, and we cultivate it even when we notice that it occasionally fails to achieve good results. If we dispensed with it, then we would require some other ritual to serve the same purpose, and it too would miscarry. Hence you don't find Marcus worrying about the existence of the gods or the importance of religion.

Ὥσπερ οἱ ἰατροὶ ἀεὶ τὰ ὄργανα καὶ σιδήρια πρόχειρα ἔχουσι πρὸς τὰ αἰφνίδια τῶν θεραπευμάτων, οὕτω τὰ δόγματα σὺ ἕτοιμα ἔχε πρὸς τὸ τὰ θεῖα καὶ ἀνθρώπινα εἰδέναι, καὶ πᾶν καὶ τὸ μικρότατον οὕτω ποιεῖν ὡς τῆς ἀμφοτέρων πρὸς ἄλληλα συνδέσεως μεμνημένον. οὔτε γὰρ ἀνθρώπινόν τι ἄνευ τῆς ἐπὶ τὰ θεῖα συναναφορᾶς εὖ πράξεις οὔτ̓ ἔμπαλιν.

Just as doctors always keep their tools and instruments at hand for unforeseen consequences of treatment, so you be sure to keep ready your actionable opinions about the affairs of gods and men. Do all things, even the slightest, with proper attention to the ties that bind divinity and humanity together, for it is impossible to achieve any human feat without some divine significance, and vice versa.