It's always so hard, working out the trail of "Doctors". Are there twelve doctors? 13? Or maybe many many more? 33, some say. Especially when the so-called "War Doctor" is considered. But this little guide may help.
The first Doctor, William Hartnell, regenerated into St Athanasius. He was martyred by the Soltarans, but regenerated into Tom Baker. Tom Baker famously wrote his "Confessions", and was then replaced by John Chrysostom. Having a golden tongue was useful in the early days of the Cybermen, as they were notoriously easy to inactivate with that element. John himself was not so happy, as he had to spend much of his active life licking Cybermen. Augustine replaced John, but spent a lot of time defending Gallifrey from the surrounding Daleks.
After Augstine's time, the whole "Doctor" concept fell out of favour. Eventually, the sequence was restored with introduction of the tough, edgy Northerner, Anselm. Hildegard of Bingen then made history by being the first female Doctor, before normality was restored with David Tennant. He survived being accused of Pelagianism, when he claimed that, by travelling backwards in time to before his own conversion, he could bring about his own Salvation.
And so we come to the present day. as the aggressive, sweary Bernard of Clairvaux threatens to be the next doctor, having made his name in "The Thick of It". One thing is for certain, as longs as their keenest fans keep turning up to conferences wearing their extravagant fancy dress (birettas, zucchettim cassocks, etc), the franchise will keep on running!