Instinctively to satisfy the needs of his subconscious spiritual desires, man needs a vertical backrest in service. From ancient times even up to now, many humans are finding out where they came from, for what are they created, who created them and where do they go after death? But unfortunately many people are trapped and lost in finding out about it, they are trapped and lost by thinking of something as God, when something is commonly unfit to be called God, but for various reasons they close their conscience leads to the truth.
All of this teaches that in discovering God it can be a long process, but common sense should always be a guide. Because rationally God must be absolute, not dependent on anything.
God equips man with a mind, compared to other creatures the human mind is the most perfect. Of course with that reason humans are able to think to distinguish what is right and what is wrong, even the main purpose of God to make a mind to humans is that humans can know God who created it. A mind is the initial capital to find God rationally.
The human mind is limited, but that does not mean that it makes us blind to find God rationally. There is an opinion that human beings can not possibly find God with their intellect because God's knowledge is so complicated, abstract and beyond human reason, this is a false opinion, because what we need to think about is not about the Essence of God but only to know how the nature -the nature of God that should be there, to sort out the true God among other gods.
Believing that God exists and recognizing his qualities can not only be accepted dogmatically but must be rationally criticized, for God completes human beings with reason.
To avoid confusion in the logic of thinking, we should be able to distinguish the following three logical conditions:
Rational - If the phenomenon is easily acceptable to common sense, something that does make sense.
Irrational - If a common sense phenomenon denotes something that is unlikely to happen, in other words, is something that is clearly absurd or impossible.
Out of Ratio - If a phenomenon can not be explained rationally because of the limitations of our mind, something that no human being ever experienced, cannot be thought of, even incapable of imagining. One example of Out of Ratio is the condition or circumstances 'outside' Space-Time.
The mistake that often occurs when thinking in finding God is to regard the actual phenomenon is clearly irrational but forced as Out of Ratio or under the pretext of dogma or it has been given so it does not need to be criticized.
Here are some of the attributes of God that must exist, without these compulsory qualities, He is not worthy of being made God.
The existence of this Universe, along with the space-time that surrounds it cannot exist by itself without a detailed and accurate Grand Design. Even as discussed at length in previous posts, it is impossible for the Big Bang event and the occurrence of some of the most accurate and harmonious natural streaks of today without the interference of supernatural forces with very high intelligence.
In connection with the creation of this accurate and harmonious Universe, an American astronomer professor, George Greenstein wrote in his book The Symbiotic Universe: "When we examine all the evidence, the ever-present thought is that supernatural powers must be involved."
(Surah: 40, Verse: 62)
No one created God, nor did God create Himself. The existence of God is not preceded by nothing and not terminated by nothing. The existence of God is absolute, not limited by Space-Time, nor is it limited to other dimensions if any.
Our universe is about 15 billion years old, as old as the surrounding Time-Space and will end in the near future or the next few billion years. Modern Sain has confirmed this. This means that our Jagat Raya is mortal, so is Space-Time. God as Creator of the Universe must be eternal. Even rationally He must be there before all is and will remain after it is over. This is possible if God's existence is not within Space-Time, since Space-Time is only the media required by matter and His other creatures for exist. God can still exist without the need for Space-Time.
Because God is outside of Space-Time then for him is the same about the past, present or future, there, here, who is far or near everything is known to him, and all in his power, Subhanallah.
"He is the First and the Last, the Ascendant and the Intimate, and He is, of all things, Knowing."
(QS: 57, Verse: 3)
Everything that exists in Space-Time, will apply the nature of Time as Before, After, Old or New, and finally will perish and vanish, because Space-Time itself will also disappear, While God whose existence is not in Space-Time, rationally will not apply the law of Time, so God will be immortal.
"And do not invoke with Allah another deity. There is no deity except Him. Everything will be destroyed except His Face. His is the judgement, and to Him you will be returned."
(QS:28, Verse: 88)
With this rationally it can be understood that everything that is regarded by man as a god, but its existence on Earth, in the sky, or anywhere in the corner of the Universe will surely perish. It is irrational that these gods can be eternal, because the gods will surely die the longest to die along with the destruction of the Universe or the collapse of Space-Time, so it would be folly if we insist on calling them God. God must be eternal, never sleep, and will not die.
Rationally the creator is different from all of His creations, different in terms of His Essence, His different qualities and also different in deeds. We and the Universe are mortal, enclosed in the medium of Space-Time, constrained by the laws of Nature.
God is the Creator, the existence of God does not require Space and not in Time, is Absolute, not limited to anything, surely His form cannot be thought of, or we imagine, Out of Ratio.
" ...,There is nothing like unto Him, and He is the Hearing, the Seeing."
(QS: 42, verse: 11)
God should be independent, free, and able to do anything, not dependent or interested in anything else, on the contrary if he still depends on something else it means that God has weaknesses.
"And whoever strives only strives for [the benefit of] himself. Indeed, Allah is free from need of the worlds."
(QS:29 , Verses: 6)
God's power will also not increase or decrease because of the deeds of His creatures. For example, if all human beings on this earth nothing is obedient to God, then God's power will not be reduced at all, nor vice versa.
If we look at this harmonious nature of the Universe, the very accurate physical phases, the strength of the balanced fundamental forces of nature, all lead to a sophisticated Grand Design, a very, very ripe plan. Then it appears that it all comes from a common source, a perfect supernatural creator, He is God, God of nature.
"And your god is one God. There is no deity [worthy of worship] except Him, the Entirely Merciful, the Especially Merciful." (QS: 2, Verse: 163)
If there are two gods, then there are times when they work together and sometimes they do not cooperate. If it works together, then there is a weakness in the two gods because they need to help each other in doing something, also help to show if the gods are absolute in power. Conversely, if they do not cooperate, this Jagat Raya will be chaotic, because many of the constants of Nature are not synchronized, the laws of nature are contradictory, then muddle the Universe.
"If in the heavens and earth there are gods other than Allah, surely the heavens and the earth will be broken.".
(QS:21, Verse: 22)
God must have absolute and unlimited power, both the power of this Universe and the power in the 'outer' Space-Time. nothing is more powerful over Him, for if there is something more powerful above Him, it is rational certainly something that is more worthy of being called God.
The absolute power also means nothing can limit it. The Space-Time dimension is one of the limits for the Material in the Universe, no Material can pass from Space-Time. So rationally God's existence must be outside of Space-Time
"Really Allâh has power over all things"
(QS:2, Verses: 20)
It is an irrational thing if anyone thinks that God can exist in this Universe as a human being or whatever it is, even the Universe whose diameter is about 30 billion years of light travel will not be able to present an infinite God.
Thus, some of the primary features of the Lord's mandate, one being logically reinforced and perfected with other Mandatory properties. By knowing the logical nature of God as above, it is hoped that we can determine and sort out the irrational gods and the rational God.
If we examine more deeply, there are many other compulsory traits of God, but with the above mentioned qualities of God, it is sufficient to be used as a guide to discovering the ultimate God, whom we are obliged to submit, to worship, and to dedicate life and die us to him.
Best Regard 
Reference :
http://www.masuk-islam.com/20-sifat-wajib-allah-dan-20-sifat-mustahil-allah-lengkap-dengan-dalil-arti-dan-penjelasannya.html
https://ardijanh.wordpress.com/2014/01/29/68/
https://ardijanh.wordpress.com/2014/02/06/bagaimana-menemukan-tuhan-dengan-cara-yang-rasional/
