If big bang cosmology is true, the universe began to exist about 15 billion years ago with the big bang. The big bang is the earliest state of the universe; "the big bang" may be taken to refer to a singularity that constitutes the first instantaneous state of the universe or (if one "cuts out" the singularity) to an explosion that constitutes the first half-open state of some brief length, e.g. the Planck length, 10-43second. In my discussion, I shall treat the big bang as a logically possible example of an earliest state of the universe.
Considerations of agent causality are not germane to our discussion; our topic is the cause of the universe's beginning to exist, not the cause of God's act of willing that the universe begin to exist. We are not examining the relation between God (the agent) and his act of willing (the effect), but the relation between his act of willing (an event) and the beginning of the universe (another event). Thus, definitions of agent causality are irrelevant to our arguments; we are interested only in definitions of event causality, where the cause and effect are both events.