I promised him that once I had a chance I would write a post about how I view lineage and lines of transmission. So, now that the book is done, here we go.
Before we deal with the issue of the integrity of a line of transmission, we should first discuss why one would want to possess such a line. I am not speaking only of Christian lines of transmission here, but of all lines claiming to pass on not only spiritual knowlege, but a direct transferance of power. These exist in Buddhism, Sufism, Hinduism, and other paths as well.
In some cases it is taught that only those who possess these lines have access to the highest states of realization. I flatly reject such notions. To quote Krishnamurti, "Truth is a pathless land". While I do believe the gates of truth can be approached through various religious paths, they are not the only way in which it is approached. Furthermore, I am a strong believer that the "ism" you approach the truth by, must eventually be abandoned if you are to enter into the Chapel fully.
You can no more take your religion into the chapel of truth, than you can take your car into your living room.
All, that said, paths and religions exist for a reason, as do lines of transmission. In general it works like this: A great Saint (Jesus, Padmasambhava, Ibn Arabi, whoever) either attains or is born with a special connection to the ultimate. They attempt to teach their students as best they can when they were alive, so that those students can pass those teachings on. In addition to those teachings they transmit a psycho-spiritual power that acts as a catalyst in co-operation with the teachings that they pass on. In the case os a Sufi Master like Ibn Arabi this is Baraka. In the case of a Tantric Master like Padmasambhava, this is Samaya transmitted through Empowerments. In the case of Jesus this is the Apostolic line of Holy Orders.
These are lineages are catalysts that enable one to do certain things in the name of the person who started the lineage. In the case of Padmasambhava, you can enter certain mandalas and generate yourself as those Yidams, and practice the associated Yogas and so on. In the case of Christ you are given the power to transmit the sacraments. I happen to enjoy both of these transmissions, and value them highly, but they are not the only way to the truth. Just a way.
Now, as far as the integrity of these lineages goes, there is some debate. Do the lines of transmission actually pass through each person that gives it, going back to the originator? Or, does each person have a direct link back to the fountainhead of the lineage?
In the case of the former we have a situation where the line can deteriorate over time, just like a message deteriorates from person to person when playing the game "telephone". This is why in Tibetan Buddhism there is a good deal of emphasis on keeping ones Samaya pure. In the Nyingma school this idea has led to Termas (hidden teachings revealed by later treasure revealers) to be more valued than Kama (teachings actually given by Padmasambhava when he was alive) because the lineage is shorter. For instance, the lineage that I recieved the Dudjom Tersar Kilaya through only has a couple jumps from me to Padmasamhava (me, Kunzang Dorje, Dudjom Rinpoche, Dudjom Lingpa, Padmasambhava). The line that I received the Kama through has dozens of people going back to the 8th century within it. Therefore the line can be damaged by being passed through people who broke vows and generally had poor spiritual worth.
Of course, not every school of Tibetan Buddhism feels this way. All the Sarma schools seem to value a teaching more the closer it is to a Sanscrit original. Thus, they value the age more than the Nyingma do. An irony, since the Nyingma are the oldest school.
In the case of Christianity this issue of whether a lineage can be corrupted is played out in the Heresy of Donatism. Donatus taught that the efficacy of a sacrament is dependant upon the moral character of the person giving the sacrament. Thus any sacrament given by a Priest who commits a serious enough sin can be considered invalid. This includes the sacrament of Holy Orders.
The Church declaired this a heresy and established a doctrine of Opere Ex Operato, or working because of the work, as opposed to Opere Ex Operantis, or working because of the worker. It is because of this rule that wandering Priests and Bishops can exist, with orders that are "Valid but Illicit". They operate outside of the doctrines of the Roman Chuch, yet possess spiritual validity.
So, in other words, even if you get the Jesus Juice from a Priest who enjoys taking the lords name in vane while nail gunning babies to trees and coveting his neighbors wife on the Sabbath, the Lineage you receieve is ultimately between you and the fountainhead of the lineage, not every person it passes through.
This is how I view it. Even in the case of the Buddhist lineages. I have worked both Kama and Terma and have found no difference between the two as far as quality of transmission. I have found Lamas who feel the same way about it as well.