Though never canonized in the Church (perhaps due to his brief foray into Montanism), Tertullian remains one of the most important early Church Fathers.

Tertullian may not have been the first to use the word, but he was the first to present a really fleshed-out idea of Trinity--using the Latin personae and substantia. His teaching of the three Persons' numerical distinction but substantive sameness in divinity is Nicene--a hundred years before the creation of the creed. However, it seems he also believed that before Creation there was no Son, as there was no necessity for the Word. This is what we call the Arian heresy.

Anyway, Tertullian experienced Christianity as a religion more of the heart than of the brain, and in opposition to Justin Martyr and philosophy famously said, "What has Athens to do with Jerusalem?"

What you should read:

De Anima
The Apology

Ratings:

(To read more about my Theologian Rating System, click HERE)
Gender Equality:

Tertullian basically hated Eve, because sin is apparently all her fault, and by extension all other women. He was all about policing their clothing (i.e. virgins should be veiled until they're married) and relationships (widows are sinning if they remarry). Apparently he didn't have much sway in this area though, which is probably a good thing.
Environmental Sensibility:

Tertullian was much more focused on the inner-life of the Christian, and said very little (or nothing) about caring for the environment.
Heretical Tendencies:

Montanism is now considered a heresy, even though many great Christians--including Augustine and current charismatic movements--have dabbled in it. Tertullian did not remain a Montanist, but it definitely had an effect on his thinking.

General Badassery:

Tertullian did a lot and wrote a lot, but not much is known of his life in general, so it's hard to say. St. Jerome wrote that Tertullian served in the African army and lived to be super old, but neither of those claims has been found true.

"He who lives only to benefit himself confers on the world a benefit when he dies."