Before Tertullian, Christians in the western half of the empire who wanted to argue theology did it in Greek. He wrote in Latin, and because there was no Latin for what he wanted to say he made it. Trinitas, the word for the doctrine, is his. So is the use of persona for the three and substantia for what they share, a formula the councils of the fourth century inherited and which survives in the creeds recited today. He also gave Latin sacramentum in its Christian sense, taking a word for a soldier's oath and applying it to the rite of initiation.
The style is unmistakable: short hammering sentences, forensic structure, a taste for the reductio and for insult. He had been trained in rhetoric and probably in law, and the Apologeticum is built like a defence speech, addressed to provincial governors, arguing that Christians are prosecuted for a name rather than for any act anyone can produce.
He is also frequently misquoted. The phrase credo quia absurdum, I believe because it is absurd, is not his. What he wrote, of the death of Christ, was that it is certain because it is impossible, in a passage about the scandal of the incarnation. The tag is later, and it has done a great deal of work standing in for a position he did not hold.