As all religious people should know, though not all behave as though they do, this world is important not for itself but as a place for growing souls. That is to say, it is designed for the building, developing and testing of human beings so that they may become fully fledged sons and daughters of God, able to function in the heavenly worlds in the fullness of love and creativity. This doesn't mean it and the things in it are not real but they are not permanent, not as they appear to be here anyway. The fact that this world is not its own justification has led some to think of it as illusion but the only illusion is to take it on its own terms. The world is real enough but it is not as it seems.
The fact that the world is an environment for spiritual growth explains much about it that puzzles us, both materialists who cannot see any sign of a divine maker, and spiritually oriented people who wonder why a divine maker would allow evil, suffering, injustice and the full panoply of negative unpleasantnesses that are so much a part of our experience. However, these test and challenge us. They bring out (or don't but could) inchoate qualities. A perfect world, such as we may assume heaven to be, would be a world in which growth, or the sort of growth we need as we are now, would not be possible. To grow a muscle you need to encounter physical resistance. To grow a soul you need to encounter spiritual resistance which is evil.
That does not mean evil is God's will, at least not evil as we currently experience it. Temporary separation from God is God's will, the separation that eventually brings about a conscious return, but evil as we know it is a spiritual aberration which can be used by God to further his purposes but was not created by God directly. It's the result of wrong choices made by beings to whom the freedom to make wrong choices so that they can consciously make right ones, as opposed to not having any other alternative, has been given.
God hides from us in this world so that we have to find him. Truth to tell, he often hides in plain sight but he is not a fact of consciousness. "Seek and ye shall find, knock and the door shall be opened" is in one sense blindingly obvious but in another it is profound wisdom. You don't find straightaway but if you persevere, thus proving the genuineness of your search and that it comes from deep within as it must if it is a search made by your whole being, then you will surely find or find enough to help you continue your search.
There is no lasting happiness in this world because there is not meant to be. If we were content with what we have we would cease to grow and so waste our lifetime. But there is also no lasting happiness here because the only thing that can really fill the spiritual hole in our heart, that can fulfil our deepest desire, is God. Nothing else lasts because nothing else is eternal. Nothing else has no limit to it. In God there is always more. He can never be exhausted.
The materialist denies God because he cannot see him but he cannot see him because he is not looking properly or with the proper organ of perception. That would be faith and imagination which in the higher aspect of these two apparently separate things is one thing. Where faith and imagination join there is truth. Faith without imagination is dry and sterile, spiritually speaking. Imagination without faith may be creative after a fashion but it is destructively so. Its products do not enhance and enrich life. They do not add to it and end up corrupting it.
Some spiritually inclined people are troubled by the darkness in the world, wondering why God would, if he is not directly responsible for it, allow it. They confuse the human self as it is in this world with the soul as it exists in the spiritual realm. The human self is an aspect of the soul but it is not the whole of it, being the portion sent to this world precisely to experience this darkness and learn to deal with it. We are not meant to be happy in this world. That doesn't mean we should never experience happiness but we should not cling to it or demand it and nor should we assume that things that do make us happy are all necessarily good. Some are, some are not. We must our judgement to discern which is which. All this is part of the learning process.
Suffering is part of life. We can moan about it, we can seek to escape it by seeking to escape that which suffers (the Buddhist approach) or we can adopt the approach demonstrated and taught by Jesus which is to accept it when it cannot be avoided (not my will but thine be done) and then refine the experience gained to develop a deeper insight into life. The face of a true spiritual person is the face of one who has suffered and allowed that suffering to work its purifying effect on the ego by giving himself over to it without resistance. This is a delicate matter. Suffering is not good but it can bring about good. This is one of the mysteries of Christianity and why turning to God does not automatically entail happiness or happiness as the world judges it.
The world is the most marvellous creation because it fulfils its function of a spiritual training ground to perfection. Yes, this world is perfect! Not perfect as in supremely good but perfect as in ideal for its purpose. The imperfection of this world points to the perfection of God.