There are a variety of different approaches and interpretations to the meaning of being 'human'. These tend to come from hundreds, possibly even thousands, of different sources, which may be readings of holy scripture, theologians' theories, analysis of texts, etc.
One argument, what Thomas Aquinas would say, is that proving the existence of God is the ability to reason. To expand on this, reasoning does not just mean being able to think for oneself, but to act instinctively and also anticipating or picturing different possible futures and consequences. Him founding the cosmological argument shows that humans use their reason to make sense of the world. Plato's chariot is a simple analogy which explains his theory on how human beings function. A charioteer, representing the 'reason', a white horse representing the 'virtue' or 'spirit', and a psychotic horse representing the 'appetite' or 'desire'. The charioteer is the most dominant figure and controls both horses in order to function. This means that without reason, a human cannot function, and therefor cannot be.
A religious argument to support reason defining a human being, is in Buddhism, one of the most practical religions. The Middle Way shows a balance between the spirit/virtue and appetite/desire. This can be seen through the teaching of the Buddha reaching enlightenment, where the Buddha had too much appetite when he lived a life of luxury in the palace and too much spirit when he practically starved himself whilst practicing being a monk. Finding the Middle Way, and therefore being enlightened, was the result of grasping his reason in himself. Furthermore, in Genesis 1:26 it is told to us to "let them (humans) have dominion". This can be interpreted that humans are only fully human when they have power. Feminists may argue this to be sexist against women, as biologically men are more physically powerful (taking power to be the meaning of dominion), and so dehumanises women for not having equal physical capabilities.
An example of not using reason is explained by Hannah Arendt, a German philosopher who lived throughout World War 2. She argues that if the German citizens would have protested against the Nazis' abuse properly, since the citizens well outnumbered the Nazis, the Holocaust would not have happened. If they had enough moral reason, the citizens were as bad as the Nazis themselves - this is called complicity. They all had a 'lack or moral imagination'. Christians would argue that the citizens weren't being human as they weren't following the command of God, the Golden Rule, "Love thy neighbour" and so weren't following the command in the 10 commandments "Do not lie", as they were lying to themselves for believing they were following the Christian religion properly.
In contrast to these reason arguments, some argue that saying being human as only the mental ability, which discriminates against those with mental disabilities and people such as young babies or very elderly people who can't reason properly. This belief reduced humanity and dehumanises people. A gruesome yet appropriate example of this being put into reality is when the Nazis killed children with mental illnesses, because they couldn't reason and so were considered unworthy of life.
Humans can be defined through comparing them to species that aren't human; a direct contrast. A reading of the poem "On Being Human" by C.S.Lewis, is that angels can't and don't experience the natural world through the five senses, whereas we do. This means that they alter their judgment and see the earth in the way God does. However, humans have the luxury of experiencing details, and only when we are dead, or to Christians resurrected to God on judgment day, can we see all the beauty.
Kantian ethics, derived from the philosopher Kant, teaches that being human means having deontological duty (doing duties because that would be morally correct). This principle depends on a priori - which may immediately convince some that Kant is wrong, as they believe that a posteriori (knowledge from experience) is more reliable - which Kant believed to be the most reliable guide to decision making. According to Kant, God made the world this way so that moral people would be rewarded in post-motern existence. Despite this, the Second Vatican Council say that "from the very onset of his history man abused his liberty, at the urging of the Evil One". This means that all humans are made in a state of holiness, but have an inclination towards evil ("evil one" meaning the serpent that appears in Genesis 2-3).
In opposition to Kant's argument, Aristotle argues that determinism is the key to being human. He says that being human is not having an inbuilt responsibility, but is reacting to what we are 'determined' to do. One may assume, especially in modern days, that we are made being able to make free choices as part of our free will. However there are factors that are out of our control that influence all our decisions in everyday life. Thus, moral decision making (which can be 'reason') is illusionary.
Some people may argue that you are human by being embodied, to whom we ascribe physical and mental characteristics. In 1 Corinthians it states that our body is the "temple of the Holy Spirit". The actual temple has the 'Holy of Holies' which has God in it. Having a body, by the Chalcedonian definition, is being 100% man and 100% God; Jesus. If God was incarnated, and had body matter, we have body matter so it is sanctified and must be "good".
I believe that being human is a triangulation of having a morality and a conscience, whether is comes in an innate way or comes from society or comes from God. I would also argue that being human requires physical characteristics of a human too, for example many other species have morality and ability to think for themselves, but this doesn't mean they are human.
