The article aims to show the way in which St Basil the Great, the bishop of Caesarea in Cappadocia sets the metaphysical basis for the Christian teaching on the relationship between God and time. St Basil has written three books against Eunomius (Adversus Eunomium), the leader of Anomean party and the bishop of Cyzicus, for the purpose of refuting the ideas presented in the latter’s book Apology. Commencing his criticism from Eunomius’ premise that the main characteristic of God’s essence is ungeneratedness (ἀγεννησία), St Basil developed the doctrine of the unknowability of the Divine essence. According to St Basil it is possible to know not the Divine essence, which always remains hidden to human and even angelic mind, but rather the Divine operations or energies through which God reveals Himself to the world. Even then this knowledge is not accurate because it consists of mere human concepts (κατ’ ἐπίνοιαν ἀνθρωπίνην) about God. In his further refutation of the teachings of Eunomius, St Basil severely attacked his view that since the Father is the cause of the Son and the first in order, it is impossible for them to share the same essence. For Eunomius the logical consequence of the causal priority of the Father is also His priority in temporal or quasi-temporal order that introduces the existence of certain period between the Father and the generation of the Son. St Basil claims that the causal priority of the Father does not necessarily imply His temporal or quasitemporal priority. Starting from Eunomius’ claims that the Son has a beginning St. Basil proposed the following argumentation against it. If the Son has a beginning, and He is not eternal, then there is a certain interval or period (διάστημα) either in time or within the endless ages between the Father and the generation of the Son. This interval between the Father and the Son can be either finite or infinite. If the interval between the Father and the Son is infinite then the generation of the Son would never take place, and therefore the interval must be finite. The fact that the interval is finite implies that it must be also measurable. If the interval is measurable, it can be measurable only in relation to the life of the Father. According to this logic the Father must also have a beginning in time, which is unacceptable. However St Basil proceeded with his critique of Eunomius further and in order to make it more convincing He engaged in an analysis of the Eunomius’ stance that the interval between the Father and the Son is not of temporal origin, but rather it belongs to the realm of endless ages. This analysis of St Basil resulted in the first systematic exposition of the Christian understanding of time. Basil defined time as an interval that coexists with creation and the beginning and the end of the world limit it. St Basil also delineated time from endless ages that are limited only by their beginning, since they were created, and the eternity, which does not have any limitations at all. This division made by St Basil opened the possibility for a radical ontological separation between the uncreated nature of the Triune God and the created nature of the world.