The Goodness of God has been clearly defined by both St. Augustine in The Nature of Good, and by St. Thomas Aquinas in the Summa Theolgica, First Part, Question Six.
St. Augustine had to address the heresy of the Manicheans, with whom he identified for years. His treatise explains that as God is good, therefore, all good things in nature are also good. We can come to know the Goodness which is God from nature.
In our day, many people believe in a dualism, which makes evil equal in power or even majesty to good. In fact, in a recent poll in the States, the fastest growing "religion", or set of beliefs, was the Star Wars sect. of young people and not-so-young, who actually believe in a Manichean form of dualism.
Here are some snippets from St. Augustine on the good.
All these things are so perspicuous, so assured, that if they who introduce another nature which God did not make, were willing to give attention, they would not be filled with so great blasphemies, as that they should place so great good things in supreme evil, and so great evil things in God. For what the truth compels them to acknowledge, namely, that all good things are from God alone, suffices for their correction, if they were willing to give heed, as I said above. Not, therefore, are great good things from one, and small good things from another; but good things great and small are from the supremely good alone, which is God.
Let us, therefore, bring before our minds good things however great, which it is fitting that we attribute to God as their author, and these having been eliminated let us see whether any nature will remain. All life both great and small, all power great and small, all safety great and small, all memory great and small, all virtue great and small, all intellect great and small, all tranquillity great and small, all plenty great and small, all sensation great and small, all light great and small, all suavity great and small, all measure great and small, all beauty great and small, all peace great and small, and whatever other like things may occur, especially such as are found throughout all things, whether spiritual or corporeal, every measure, every form, every order both great and small, are from the Lord God. All which good things whoever should wish to abuse, pays the penalty by divine judgment; but where none of these things shall have been present at all, no nature will remain.
Magnificently and divinely, therefore, our God said to his servant: "I am that I am," and "You shall say to the children of Israel, He who is sent me to you." Exodus 3:14 For He truly is because He is unchangeable. For every change makes what was not, to be: therefore He truly is, who is unchangeable; but all other things that were made by Him have received being from Him each in its own measure. To Him who is highest, therefore nothing can be contrary, save what is not; and consequently as from Him everything that is good has its being, so from Him is everything that bynature exists; since everything that exists by nature is good. Thus every nature is good, and everything good is from God; therefore every nature is from God.
Another popular idea is that evil comes from God as well as good, an idea held by millions in a certain false belief system.
Aquinas writes this:
God is the supreme good simply, and not only as existing in any genus or order of things. For good is attributed to God, as was said in the preceding article, inasmuch as all desired perfections flow from Him as from the first cause. They do not, however, flow from Him as from a univocal agent, as shown above (Question 4, Article 2); but as from an agent which does not agree with its effects either in species or genus. Now the likeness of an effect in the univocal cause is found uniformly; but in the equivocal cause it is found more excellently, as, heat is in the sun more excellently than it is in fire. Therefore as good is in God as in the first, but not the univocal, cause of all things, it must be in Him in a most excellent way; and therefore He is called the supreme good.
All goodness, great and small, come from He Who Is Goodness and the Supreme Good. God is good in His Essence, which makes Goodness an Attribute of God.
Aquinas explains that all that is good has its origin in God.
Although this opinion appears to be unreasonable in affirming separate ideas of natural things as subsisting of themselves--as Aristotle argues in many ways--still, it is absolutely true that there is first something which is essentially being and essentially good, which we call God, as appears from what is shown above (Question 2, Article 3), and Aristotle agrees with this. Hence from the first being,essentially such, and good, everything can be called good and a being, inasmuch as it participates in it by way of a certain assimilation which is far removed and defective; as appears from the above (Question 4, Article 3).
Everything is therefore called good from the divine goodness, as from the first exemplary effective and final principle of all goodness. Nevertheless, everything is called good by reason of the similitude of the divine goodness belonging to it, which is formally its own goodness, whereby it is denominated good. And so of all things there is one goodness, and yet many goodnesses.
Too often, today, young people fall into a cynicism about goodness, denying or doubting that there exists any goodness in the world. These two great Doctors of the Church assure us of God as Good.
We can participate in the goodness of God only in so far as we allow Him to cleanse us of evil. Our goodness would be from that of God, and made in His image and likeness, we have lost the likeness, which is grace, according to St. Bernard of Clairvaux and must regain that likeness through the sacramental life of the Church and through purification.
Some can get into a mental habit of concentrating on evil rather than good. We become like that of which we think and that which we love.
If we love Goodness, we shall regain our likeness, which we lost in the beginning of humankind.