God was not created.
If something is created, then it is necessarily temporal -- in other words, it changes over time. Specifically, it must begin to exist, and "beginning" is itself a temporal process.
However, since the only things that are infinite are things that have no end, and since past time has ended, we can say with certainty that past time is not infinite, which means that past time had a beginning.
Whatever initiated time must be non-temporal, or eternal, or else it would not have been able to initiate time. Being eternal, it must therefore be uncaused. Since it initiated time, it must be the first cause, as no cause can come before it, since the word "before" is meaningless outside of time. As the eternal uncaused first cause, it could not have been forced to initiate time (that would mean that something caused it to cause, putting a cause before it, which we've already seen is impossible), and therefore it must have acted voluntarily and volitionally, making it necessarily a personal entity. So we necessarily recognize an eternal, personal, uncaused first cause because of the impossibility of the contrary.
In other words, because time passes, we know that God is uncreated.
More importantly, the Bible tells us that God Himself is uncreated:
All things came into being through Him, and apart from Him nothing came into being that has come into being.
For by Him all things were created, both in the heavens and on earth, visible and invisible, whether thrones or dominions or rulers or authorities -- all things have been created through Him and for Him. He is before all things, and in Him all things hold together.
So we see that God is uncreated because the contrary is impossible. This is confirmed by both scripture and logic.