The concept of a 'perfect god' contradicts our idea of God's personality and drives. If God is perfect, then, it needs nothing. It cannot want anything, because a want denotes a lack, and a perfect being is short of nothing. If god exists in a perfect state, then, change is impossible. Any change towards a new state implies that god's original state was not the perfect one, after all. So, a perfect god is immutable and does not change1,2. But an unchanging being has no emotions, no feelings, no 'plans' as such - all of its plans existed in their perfect state right from the beginning; co-created in the same manner as god itself. With no changing emotions or desires, a perfect god is a frozen automaton, with its perfect plan rolling out whether or not it acts or thinks or moves or feels. The concept of a perfect god also contradicts reality: creation is not perfect and the existence of suffering, violence and other negative aspects means that god is not benevolent and perfect, at least not according to moral or functional definitions of "good" or "perfect".
If God is perfect, then, it needs nothing. It cannot want anything, because a want denote a lack, and a perfect being is short of nothing. If a God is perfect, it knows everything (omniscience); therefore it is never surprised, never needs to 'find out' or 'test' or 'search', and never 'wonders'. It knows.
William James calls this the lack of 'potential' for God:
“This absence of all potentiality in God obliges Him to be immutable. [...] Were there anything potential about Him, He would either lose or gain by its actualization, and either loss or gain would contradict his perfection. He cannot, therefore, change.”
“Where there is life there is change or Transience. Where there is more change there is more vital activity. Suppose an absolutely changeless body: it must be absolutely lifeless. An eternally changeless life is equivalent to an eternally changeless death.”
"Zen - The Religion of the Samurai" by Kaiten Nukariya (1913)2
The Christian Bible in Matthew 5:48 says that God is perfect, but, Thomas Paine makes the criticism that God creates an entire new religion after changing its mind about Judaism3; likewise, God changes its mind throughout the Bible as events unfold: but to change its mind is to admit a mistake or a lack of knowledge (or justice), and therefore, the Biblical God isn't perfect.
To be an eternal being that is responsible for creating the flow of time itself, is to be immutable and unchanging. God has existed for all eternity before the creation of the world, and all of eternity after the demise of the world. The created world - from beginning to end - is like an object in the hands of God, that can be rotated and examined. God can view every timeline from start to finish, and knows the conclusion of every test. This First Cause of the Universe sits on the outside of time, looking it, effectively omniscient. But God is not subject to the laws of the Universe that it created. It holds all of time and space in its hands but it is not itself subject to time. For this reason, God doesn't change. And for another reason, too: God is a perfect being. Any change away from a perfect state in order to achieve some aim or goal must be a step towards a good purpose: but, god, in being perfect, has already attained all good ends. God doesn't consist of an eternal series of mental states: God is one mental state, unchanging, perfect and eternal1.4
The conclusions that God is unchangeable has been reached by some of the leading historical Christian theologians. St Thomas Aquinas in the second book of his Summae Theologiae concludes that god cannot change itself, or repent, or undo the past5. This is because these things are temporal events that require God to be subject to time itself; but as time is a dimension created by god, God is above and outside it5. The Bible supports this in Malachi 3:6, Numbers 23:19 and James 1:17.
The result is that God is immeasurably cold and emotionless2; much more like an automatic process, rather than like the God that many people wish exists. It seems the very concept of God verges on being self-contradictory: It is more of a principle, unconscious and non-thinking. These and other theological problems have led many to the conclusion that God simply doesn't exist.
For more, see:
A powerful argument against the existence of a perfect and benevolent god is the existence of suffering, violence and death. A perfect God would have created a perfect world.
If God is all-powerful and all-good, it would have created a universe in the same way it created heaven: with free will for all, no suffering and no evil. But evil and suffering exist. Therefore God does not exist, is not all-powerful or is not benevolent (good)6. Such arguments have been used by many philosophers as evidence against belief in god7,8. A theodicy is an attempt to explain why a good god would have created evil and suffering. The most popular defence is that it is so Humans could have free will. However the entire universe and the natural world is filled with suffering, violence and destruction so any Humanity-centric explanation does not seem to work.
For more on this topic, see The Problem of Evil: Why Would a Good God Create Suffering?: