If God is truly outside time and unchanging, He cannot have created the world, answered a prayer, or become incarnate — all of which are acts in time.

Why people find it persuasive

Acting seems to inherently involve a before and after — deciding, then doing — and if God is genuinely timeless with no sequence of states at all, it is hard to see how any specific act (like creating at a particular moment, or the incarnation happening in first-century Judea) could be attributed to Him without introducing temporal sequence into God after all.

Important distinctions

• • God's intrinsic nature (atemporal and unchanging) vs. God's relation to a temporal creation (which can begin at a point without God Himself undergoing change).
• • A change in God's inner being vs. a change merely in relational or extrinsic properties (e.g., becoming 'Creator' the moment creation exists).
• • Timelessness prior to creation vs. a view (sometimes called the 'timeless-then-temporal' model) on which God enters time at creation.
• • Metaphysical action as decision-then-execution vs. a single eternal timeless act with temporal effects at various points.

Christian response

• • Present the classical (Boethian/Thomistic) model: God's single, eternal, timeless act of will has effects that occur at particular points within time, without that act itself being sequential from God's own perspective — much as a single unchanging cause can be correctly described as bringing about a temporally ordered series of effects (a common analogy is an eternal footprint impressed by an eternally-standing foot, where the impression's temporal existence doesn't require sequence in the cause).
• • Distinguish intrinsic change (a real alteration in what God is) from what philosophers call a mere 'Cambridge change' — a change only in relational description. When creation begins, God does not undergo any intrinsic alteration; He merely comes to stand in a new relation (Creator-of-x) to something that now exists, similar to how a person can become 'shorter than' someone else purely because the other person grew, without the first person changing at all.
• • Note the live alternative view within serious Christian philosophy: philosophers like William Lane Craig defend a model on which God is timeless without creation but enters into time at the moment of creation (in virtue of really being related to a changing temporal world), avoiding any incoherence by giving up strict timelessness after creation rather than trying to fit all temporal action into an atemporal frame.
• • Point out the incarnation is handled by classical Christology as the eternal Son taking on a human nature that itself exists and acts temporally, without requiring the divine nature itself to become temporal — the sequence and change belong properly to the assumed human nature, following the same person-vs-nature distinctions worked out in Trinitarian and Christological doctrine.
• • Acknowledge this is a live and technical topic in analytic philosophy of religion (with serious, mutually respectful disagreement between eternalist/Boethian and temporalist/Craig-style models among orthodox theists), rather than a solved settled question, and different resolutions are available depending on which model of divine eternity one adopts.

Christian counterresponse

Defenders respond that 'deciding' and 'responding' need not name sequential mental events even for God; they can name a single timeless act that is appropriately correlated with, and responsive in content to, temporal circumstances (including free creaturely prayers, foreknown timelessly), so what looks like sequence in ordinary human action-language does not have to be read univocally onto divine action.