We can only partially comprehend the notion of God's existence. To do so, we must use human concepts to speak of God: "without beginning or end"; "eternal"; "infinite", etc. The Bible says that He has always existed: " . . . even from everlasting to everlasting, thou art God" (Psalm 90:2). And, "Your throne is established from of old; Thou art from everlasting" (Psalm 93:2). Quite simply, God has no beginning and no end. So, where did God come from? He didn't. He always was.
To us, the notion of time is linear. One second follows the next, one minute is after another. We get older, not younger and we cannot repeat the minutes that have passed us by. We have all seen the time lines on charts: early time is on the left and later time is on the right. We see nations, people's lives, and plans mapped out on straight lines from left to right. We see a beginning and an end. But God is "beyond the chart." He has no beginning or end. He simply has always been.
Also, physics has shown that time is a property that is the result of the existence of matter. Time exists when matter exists. Time has even been called the fourth dimension. But God is not matter. In fact, God created matter. He created the universe. So, time began when God created the universe. Before that, God was simply existing and time had no meaning (except conceptually), no relation to Him. Therefore, to ask where God came from is to ask a question that cannot really be applied to God in the first place. Because time has no meaning with God in relation to who He is, eternity is also not something that can be absolutely related to God. God is even beyond eternity.
Eternity is a term that we finite creatures use to express the concept of something that has no end -- and/or no beginning. Since God has no beginning or end, He has no beginning. This is because He is outside of time.
