My friend Morgan Guyton points out: ‘when Paul uses the term haeretikos in Titus 3:10 that gets translated into our term “heretic,” the problem with the person he’s describing is not the incorrectness of his views, but his divisiveness.
Ironically, the greatest heretic-hunters are usually themselves the greatest heretics if we are using the word the way that the apostle Paul used it.’
The same point Morgan makes from Paul can be made by way of St Thomas Aquinas.
As I’ve been hammering here on the blog lately, Aquinas’ central thesis in his Summa Theologica is that the God who reveals himself in the Burning Bush (‘I Am He Who Is’) is not a god among the gods of the world. God is not an object within the material universe. God is not a kind (Thomas uses the word ‘genus’) of being alongside other beings like you or me.
God is the Source of Being. It’s because of God that anything from quarks to quacking ducks is at all. It’s because of God that there are somethings instead Nothing.
Everything that is is because of God, and that includes everything that happens, all our thoughts and deeds and decisions exist because of God.
A song relies upon the singer to keep it in being; likewise, God is the cause of all things in every moment.
As Hebert McCabe elaborates on Thomas:
“It is quite a thought that if you choose to break the law of God by cruelty or indifference to suffering, it is the Lord who is keeping you in existence while you are doing this, from second to second.
To think you are defying the Lord is the ultimate absurdity and contradiction, for you only exist, you only are because of God.
This self-delusion, the delusion that you can stand over against God, that you are not a creature- this is what sin is.”
It’s worth repeating in case you skipped past the quote: to think you can defy God, who is at every moment the cause of your being at all, is the definition of sin. 
It’s to deny a creature-hood which entails God’s constant sustaining. It’s to imagine we’re something more than creatures- to imagine we’re gods, free and independent of God except when we call upon the great cosmic butler.
And when see ourselves as already more than creatures, we forget the meaning of salvation: that God aims to take us beyond our creature-hood in the Son and through the Spirit into the life of the Trinity.
If imagining that we can defy God is the most basic and thus the most absurd of all sins, then ironically…
it’s those of us who obsessively point out other people’s sins- those people we perceive to be defying God- who are the worst sinners of all.
We look at people and refuse to see them for what they are: creatures held in being by God at every moment- every moment- of their lives.
In other words, Thomas simply points out what we learn from the Elder Brother’s refusal to join the Father’s Feast (Luke 15).
Sin = Refusal of Grace.
