From The Fathers of the Church, by Mike Aquilina, pg 92 - 93
Inspired by the courage of Christian martyrs, Tertullian converted to the faith when he was in his late thirties. [snip] Once in the Church, he wielded his pen like a blazing brand, to expose error by the light ot truth (one of his favourite words), and to immolate falsehood with the flames of his invective. It was Tertullian who first used the Latin word Trinitas (Trinity) to describe God.
Problems arose when he encountered infidelities, apostasy, cowardice, lukewarmness and immorality among his own people. He came, more and more, to excoriate those who called themselves Christian but who fell into mortal sin. Calling for a purer Church, he fell under the influence of the schismatic Montanus, who claimed to speak by the power of the Holy Spirit. In time, Tertullian came to invent a distinction between the "spiritual church" and the "church of a bunch of bishops." He joined the Montanist sect - though, ultimately, he would find even them unsatisfactory. He then founded his own sect, the Tertullianists, which would survive till St Augustine's day.
Get the book if you haven't. It makes for captivating reading, to learn about how the early Church was like, and to discover many parallels with the modern Church today.