1. Either (a) the purposes God sets for our lives are significant because God wills them, or (b) God wills them because they're significant.
2. If (a), then what counts as a significant life is arbitrary.
3. If (b), then what counts as a significant life is independent of God
4. Therefore, what counts as a significant life is either arbitrary or independent of God.
Perhaps the theist could go between the horns of the dilemma by saying that (i) God is a perfect being, (ii) whatever is a perfect being is the ultimate source of value, and (iii) humans and other beings and their activities/projects are valuable by virtue of resembling God and his activities/projects. But then it seems that sort of reply is likewise subject to a Euthyphro-style objection:
1. Either God and his activities/projects have value in virtue of having the properties that constitute perfection, or the properties that constitute perfection have value in virtue of God and his activities/projects having them.
2. If God and his activities/projects have value in virtue of having the properties that constitute perfection, then those properties are the standard of value -- not God and his activities/projects.
3. If the properties that constitute perfection have value in virtue of God and his activities/projects instantiating them, then the standards of value are arbitrary.
4. Therefore, either the properties that constitute perfection are the standards of value (and not God and his activities/projects), or the standards of value are arbitrary. (cf. Morriston 2001)
The theist has another trick up their sleeve. Thus, they might appeal to a feature of Robert Adams' (1999) version of modified divine command theory. In particular, they might reply that God is the ultimate source and standard of value, and that God's being confers value on his properties and activities/projects (e.g., his omnipotence, omniscience, perfect goodness, lovingkindness, and his activities and projects). On this sort of view, when it comes to value, God functions in a way analogous to the standard meter stick in Paris. Thus, while the length of the standard meter stick is not analyzed in terms of a standard of goodness beyond itself, the stick serves as the standard by which all other lengths have the property of being a meter in length: An object is a meter in length just in case its length exactly resembles the length of that stick. Similarly, while God (qua the Good) is not analyzed in terms of any other standard beyond himself, he serves as the standard by which all other entities have the property of goodness: An object, property, activity, or project is good/valuable just to the extent that it resembles that being and his activities/projects (viz., God qua the Good). On this sort of view, then, goodness supervenes directly on God’s being and confers goodness on his loving, kind, honest, just, etc., nature, as well as his activities and projects
Two main criticisms have been raised against the latter sort of view. First, the view entails that it gets the relevant counterpossibles wrong: If God didn’t exist, then kind, honest, loving, just humans wouldn’t be good, which is implausible. (Morriston 2001). Second, since God is not good in virtue of being kind, loving, honest, just, etc., the view makes it unintelligible what God’s goodness/value consists in (Morriston 2001; Koons 2012). Similar points apply to claims about Adams-inspired accounts of meaningful lives and activities.
Here's another way to get at the point. Suppose we ask what is so special about being made in God's image. Presumably it's that he's a person, and that persons have intrinsic value. (Leave to the side the issue that other things besides persons seem to have intrinsic value.). But then it follows that, necessarily, anything that instantiates personhood instantiates intrinsic value, in which case human beings would have value and significance whether or not God exists.
Similarly, suppose we ask what it is about God's own life purposes and activities that makes them significant and worthwhile for Him? Presumably, such activities are intrinsically worthwhile for persons to engage in (e.g., pursuing, engaging in, and sustaining transparent, caring relationships; creating beautiful things; contributing to the welfare of others; gaining and sharing knowledge; etc.). But if these activities are intrinsically worthwhile and significant for persons to engage in, then they are so independently of whether God exists or sets them for us.
The takeaway: As with the roots of morality, the roots of meaning in life go deeper than God.
Adams, Robert. 1999. Finite and Infinite Goods: A Framework for Ethics. Oxford: Oxford University Press.
Koons, Jeremy. 2012. “Can God’s Goodness Save Divine Command Theory from
the Euthyphro?” European Journal for Philosophy of Religion 4 (1): 177–195.
Morriston, Wes. 2001. “Must There Be a Standard of Moral Goodness Apart
from God?” Philosophia Christi (Series 2) 3 (1): 127–138.