I thought it would be interesting if I put this question in relevant historical philosophical contexts and give an account of important views. So it would be a long but very enlightening discussion I believe.
Abstract can mean a number of things. It may be something that is a product of imagination without real existence. Take “unicorn” for example.
In philosophical parlance, it may denote universal concepts that refer to concrete objects, such as man, animal, plant, etc -- as they are thought to be "abstracted" (i.e. separated) from their concrete instances -- or alternately to “pure concepts”, such as the ones you named, e.g. eternity, happiness, love, and unity, existence, and cause, to add more.
There’s a historical debate about the origin and nature of these concepts, i.e. whether universals exist independently of the mind and whether this is true for pure concepts too. One may consult Stanford entry on Abstract Objects, Nominalism and Platonism.
The modern popular view is that all these concepts don’t have an independent existence, they are all mental abstractions from concrete objects. However “pure concepts” are more tricky as they don’t have concrete instances in the outside world but only referents. That is to say that you can’t show unity and cause for example in the outer world but you use them to describe relations between instances of universals such as fire and heat with fire being the “cause” of heat, or their analytic aspects such as red being “a property” or “accident” of fire. So again the question would be if these analytic/relational aspects are real in the sense that they describe a true state of affairs in the world, what do we say exactly of the concepts themselves that describe these aspects? Is “cause” or “property” things that exist independently of fire, red and heat in these examples? How exactly the concepts relate to the observed phenomena and when we observe a relation or an analytic aspect what exactly do we observe beyond some concrete objects in a particular context and configuration?
This is a mind boggling problem and continues to preoccupy minds of thinkers. However the popular view has origin in Immanuel Kant's philosophy which effectively reduces pure concepts to forms in human mind that are kinda imposed on sensual observations by it. The imposition would be scientifically valid if they are confirmed by repeated observation. But on their own, i.e. independent of the phenomena that they apply to, they are not significant. Therefore, if there are abstracts for which there is no observable phenomena to describe they have no epistemic value. Kant considered God, soul, immortality, freedom, goodness, etc to be such concepts. However he argued since we can’t live without presuming at least some of these concepts such as freedom (as it is the basis of ethics), he argued that they must be taken as granted not because they are real but because it is pragmatic to do so! In other words, they can be thought of as useful illusions! (As for God and immortality he didn't think they had even pragmatic value; man can live a moral life without belief in religion).
This idea of useful illusions might be a strange position but it also represents the biggest philosophical problem that man appears to have stumbled in history of philosophy. The problem continues to affect many modern scientific questions, such as the persisting debate in philosophy of mind on whether human consciousness is a real thing or simply an illusion caused by physical brain activity.
Contrary to the modern popular view, is a traditional view that has historically appealed to religions, which holds that universals, and probably pure concepts too, do independently exist but on a postulated higher plane of existence, the Platonic “World of Ideas”. According to this view these ideas are actually even more real than the observable objects that they refer to because natural phenomena are in fact only a vague and imperfect image of these ideas that exist as very substantial transcendent objects!
In history of Islamic theology, this latter view was adopted by many Muslim theologians, particularly the Sufi thinkers or philosophers influenced by Sufism.
The idea is that since Divine attributes are also analytically abstract — we talk about Allah’s awareness, justice, love, eternity, etc without any concrete reference — they can’t be illusions.
The argument by Muslim Sufi meta-physicians goes like this: first they argue for a dualist position on the mind-body problem: the very occurrence of pure concepts in our mind is evidence of mind's immaterial nature. Second, even though these concepts don't have concrete objects, they have a concrete sense or feel in our mind/heart which happens to be the most intimate and concrete form of knowledge we have of anything in the world. We all indeed can sense attributes such as happiness, justice, wisdom and love in our being independent of their physical correlates or referents. And now since these inner qualities really direct our physical body and since they are categorically distinct from physical attributes, they indicate a higher dimension wherefrom they originate. Their ultimate origin is attributes in God where all these qualities exist in a perfect and absolute mode. That's why we should actually seek these qualities in Allah:
لِلَّهِ الْأَسْماءُ الْحُسْنی فَادْعُوهُ بِها
For Allah is nice names, so call Him by them. (Quran, 7:180)
It is needless to say, that Divine names are not like names that humans arbitrary put on objects or persons. They indicate real attributes in Allah and that they can be recognized and realized by man, otherwise there would be no point in Allah mentioning them and there would be no way of us relating or communicating with Allah anyway.
Conclusion and Summary
So yes, concepts like Allah and happiness as any other concepts even man and tree can be ideas in our mind. But their actual existence is not found in the same dimension of reality. For physical concepts you usually look without, but for supernatural and some "abstract" concepts you look deep within! But an atheist usually can't acknowledge their existence and/or substance because they have not developed any inner connection to Allah so as to sense how these qualities can be felt with greater and greater intensity and freshness when a faithful heart invokes them during ritual and prayer. They only have the minimum conceptual sense of these ideas that is shared by all minds but not their concrete feel in our hearts that testify to their substance and transcendent origin.
Let me conclude by this brilliant verse of the Holy Quran which can't be any more relevant:
سَنُرِيهِمْ آيَاتِنَا فِي الْآفَاقِ وَفِي أَنْفُسِهِمْ حَتَّى يَتَبَيَّنَ لَهُمْ أَنَّهُ الْحَقُّ
And we will show our sings in the horizons and within themselves so
that they realize that He is real. (Quran:41:53)