God's attributes known negatively
Following St. Damascene (De Fide Orth. i, 4), St. Thomas Aquinas writes (Summa Theologica I q. 2 a. 2 arg. 2):
we cannot know in what God's essence consists, but solely in what it does not consist
This is called apophatic theology; ἀποϕατικός = negative.
This is the manner in which we know the divine attributes by natural reason:
Summa Contra Gentiles I c. 14 "That to know God we must use the way of remotion" (i.e., proceed by saying what God is not):
We have shown that there exists a first being, whom we call God. We must, accordingly, now investigate the properties of this being.
Now, in considering the divine substance, we should especially make use of the method of remotion. For, by its immensity, the divine substance surpasses every form that our intellect reaches. Thus we are unable to apprehend it by knowing what it is. Yet we are able to have some knowledge of it by knowing what it is not. Furthermore, we approach nearer to a knowledge of God according as through our intellect we are able to remove more and more things from Him. For we know each thing more perfectly the more fully we see its differences from other things; for each thing has within itself its own being, distinct from all other things. So, too, in the case of the things whose definitions we know. We locate them in a genus, through which we know in a general way what they are. Then we add differences to each thing, by which it may be distinguished from other things. In this way, a complete knowledge of a substance is built up.
However, in the consideration of the divine substance we cannot take a what as a genus; nor can we derive the distinction of God from things by differences affirmed of God. For this reason, we must derive the distinction of God from other beings by means of negative differences. And just as among affirmative differences one contracts the other, so one negative difference is contracted by another that makes it to differ from many beings. For example, if we say that God is not an accident, we thereby distinguish Him from all accidents. Then, if we add that He is not a body, we shall further distinguish Him from certain substances. And thus, proceeding in order, by such negations God will be distinguished from all that He is not. Finally, there will then be a proper consideration of God’s substance when He will be known as distinct from all things. Yet, this knowledge will not be perfect, since it will not tell us what God is in Himself.
Divine Attributes
God is not
material → divine simplicity (I q. 3; SCG I c. 17)
lacking anything (having passive potentiality) → God is perfect (I q. 4; SCG I c. 16)
limited → infinity of God (I q. 7; SCG I c. 43)
changeable → immutability of God (I q. 9)
bound in time → eternity of God (I q. 10)
multiple/composite → unity of God (I q. 11; SCG I c. 42)
ignorant → knowing (SCG I c. 44)
in a genus (SCG I c. 25)
etc.
Also, Fr. Hardon, S.J., defines "divine attributes" as
The perfections of God, which, according to a human way of thinking, proceed from and belong to the essence of God. In reality the divine attributes are identical among themselves and with the divine essence. Theology distinguishes the attributes from the essence because they correspond, in human language, to different properties in creatures which reflect, so to speak, the perfections of God.
God known by analogy of proportion, not attribution
Summa Contra Gentiles I c. 34, "That names said of God and creatures are said analogically":
From what we have said, therefore, it remains that the names said of God and creatures are predicated neither univocally nor equivocally but analogically, that is, according to an order or reference to something one.
This can take place in two ways. In one way [analogy of attribution], according as many things have reference to something one. Thus, with reference to one health we say that an animal is healthy as the subject of health, medicine is healthy as its cause, food as its preserver, urine as its sign.
In another way [analogy of proportion], the analogy can obtain according as the order or reference of two things is not to something else but to one of them. Thus, being is said of substance and accident according as an accident has reference to a substance, and not according as substance and accident are referred to a third thing.
Now, the names said of God and things are not said analogically according to the first mode of analogy [of attribution], since we should then have to posit something prior to God, but according to the second mode [analogy of proportion].
In this second mode of analogical predication the order according to the name and according to reality is sometimes found to be the same and sometimes not. For the order of the name follows the order of knowledge, because it is the sign of an intelligible conception. When, therefore, that which is prior in reality is found likewise to be prior in knowledge, the same thing is found to be prior both according to the meaning of the name and according to the nature of the thing. Thus, substance is prior to accident both in nature, in so far as substance is the cause of accident, and in knowledge, in so far as substance is included in the definition of accident. Hence, being is said of substance by priority over accident both according to the nature of the thing and according to the meaning of the name. But when that which is prior in nature is subsequent in our knowledge, then there is not the same order in analogicals according to reality and according to the meaning of the name. Thus, the power to heal, which is found in all health-giving things, is by nature prior to the health that is in the animal, as a cause is prior to an effect; but because we know this healing power through an effect, we likewise name it from its effect. Hence it is that the health-giving is prior in reality, but animal is by priority called healthy according to the meaning of the name.
Thus, therefore, because we come to a knowledge of God from other things, the reality in the names said of God and other things belongs by priority in God according to His mode of being, but the meaning of the name belongs to God by posteriority. And so He is said to be named from His effects.