Beauty defined may be abstract and remote; beauty described should be concrete. What does beauty look like? For that matter, since beauty is not only (or even primarily) visual, what does beauty sound like, feel like, or seem like? When we have encountered or experienced beauty, what is that experience?
The best approach is to start from the deepest reality and work outwards. If beauty is the ultimate ground of reality, the self-giving love of the Godhead to one another, then beauty in the created order will be a form, or an echo of that primary beauty.
Among rational, sentient beings, this beauty is seen in self-giving love. When free beings choose to willingly unite their wills with the good of others, this is the closest echo of Trinitarian beauty. Symmetry of wills is the greatest symmetry of all. No wonder that the Greatest Commandment is to love God wholeheartedly (unite our wills entirely with His glory), and the Second is like it, to love our neighbour as oneself (unite our wills with what will beautify and bless our neighbour). Those who live in this kind of love are simultaneously growing in knowledge of truth and goodness (Phil. 1:9-11). Furthermore, the more they pursue this beauty, the more they are “beautified” by it, and become rooted and grounded in, enabling even further knowledge of it (Ephesians 3:16-19). And when this is the case, the soul being shaped into the beauty of God finds ultimate pleasure and joy in knowing and loving God (Ps 34:4). Harmony, symmetry, pleasure, truth and goodness (all the suggested definitions of love) find their manifestation in God’s love, considered objectively or subjectively.
When we move out from sentient beings to created material, beauty continues to be seen when it represents the harmony of wills. Whether it is the arrangement of sound, colour, shapes, numbers, words, or ideas, beauty is found in the combination and arrangement of disparate parts to make a unified whole. Whether found naturally, or re-shaped by God’s image-bearers, created beauty is a re-enactment of Genesis 1:2-3: order, harmony, equality is brought to bear upon what is without form and void, so that we can make the evaluation: “it is good”. Of course, that description of created beauty would need a lot of nuancing and explaining, given the complexity and variety in the created order. But it appears “approving what is excellent” is precisely part of the task of the righteous in this age.
But here’s the rub. Since beauty is ultimately an echo of holy love, the moral state of the beholder influences the perception of beauty. If evil is a distortion of God’s love, then those who love to do evil have come to find pleasure in what God calls ugly. Or to put it another way, for evildoers, beautiful has become ugly, and ugly has become beautiful (Is. 5:20). This explains why Jesus says the real condemnation of man is not his ignorance, but his deliberate avoidance of God’s beauty and the contrast it will make with man’s ugliness (John 3:19-21). By contrast, the first mark of regeneration is a taste or relish for God’s beauty. Sanctification is a steady process of learning to hate what is unlovely to God, and love what God loves.
This does not mean that unbelievers cannot perceive or create beauty. As image-bearers, they can perceive truth, goodness, and beauty. They can find pleasure in it, depending on how much free reign they have given to their depravity (few reach the place of total demonic hatred of all that is good). And to the degree that they mimic (wittingly or unwittingly) the self-giving nature of love, they may both make beautiful things, and recognise beautiful things.
But as the beautiful thing begins to bear a more recognisable resemblance to God, conviction sets in, and the unbeliever will wish to flee. Only the overcoming grace of God will infallibly persuade and bring the fleeing unbeliever to finally embrace the source of beauty: God in Christ.