[HarleyD]

Most of the Fathers of the Church I am aware of are the ones quoted in Catholic Prayer books or books on theology. I always assumed the Fathers of the Church would also have to be declared Saints. Guess I'm wrong. Is there a criteria for determining who is and isn't? I would think they all had to be from the 6th century or earlier

That is a good question. You can read the church fathers here. I've seen St. Anselm referred to (somewhere) as a Church father but he lived much later than the 6th century. This is a rather confusing issue in my mind.

I would agree with you that I believe the true church fathers lived from the 6th century and earlier. I wouldn't be so quick to dismiss their writings if they were not "Saints" simply because there is much information to be gleaned from their writings. The Catholic Church even admits that Tertullian doctrine and writings for the most part were good although he was at odds with the Church. Saint or not, all their writings do not raise to the same level as scripture according to Church teaching.

[kosta50]

The process of canonization of a saint is different in the West then in the East (the latter never changed). In the East, a man or a woman is canonized when the laity begin to venerate him or her spontaneously. In the West, the proces sinvolves Church clergy and proofs of miracles.

This is, of course, very grossly simplified and only FYI.

[MarkBsnr]

Tertullian became a heretic over time. He gradually drifted from orthodoxy.

It’s not that he was ‘good’ only a portion of the time; for a long while he was within the Church and its teachings.