Why did celibacy become mandatory for priests?

Let me start by copying and pasting an answer to a similar
question received previously. You will find the rest of my answer to your
specific question after the following:

Can you tell me when the Catholic church started insisting its priests
were celibate?

How celibacy became common and why it became required are two very
different things. In the third and fourth centuries it became common in
primitive Christianity for men to choose asceticism as a way of expressing
their devotion to God. Asceticism is a general term to describe the
philosophy that one becomes close to God through radically denying oneself
the pleasures of life. Asceticism throughout history has included
celibacy, ritualistic fasting, cloistering (removal from all outside human
contact in monasteries), willful injury of oneself (walking on knees many
miles, cutting oneself in places where Jesus was wounded, etc.), becoming
a hermit, purposefully wearing only rags,… The list could go on.
Celibacy has certainly not been limited to Christianity. The Essenes were
an ascetic Jewish sect active at the time of Jesus Christ. Hinduism,
Buddhism and especially Jaina are world religions which have inspired many
to turn to asceticism.

Getting back to Christianity, there was a strong movement in
Christianity toward asceticism by the third century AD. This was
especially dominant in the North African churches. Many, in their
misguided, but perhaps sincere desire to become godly chose to become
hermits, to practice celibacy and so forth. By the fourth and fifth
centuries, orders of monks became common, including vows of celibacy and
poverty.

During this time, one might be surprised that celibacy amongst
priests was not overwhelmingly common. In fact, in the eighth ninth and
tenth centuries AD, polygamy and concubinage was very common amongst Roman
Catholic priests. Eventually, Rome even sanctioned catholic priests
taking a second wife if their first became sick. However, this clear
violation of biblical teaching led to a problem. With so many children of
priests running around, inheritance became a huge problem. In an effort
to protect church property, Pelagius I required all priests to sign an
agreement not to allow any of their children to inherit church property.
In AD 1022 Pope Benedict VIII officially declared that priests were banned
from taking on wives or mistresses in order to protect church property
rights, although those who were married before entering the priesthood
were allowed to keep their wives. Finally, in AD 1139, Pope Innocent II
declared all priest marriages annulled, declaring celibacy the rule for
all Catholic priests from that day forward. To be fair to Innocent II,
his reforms were at least as much directed toward eliminating the blatant
sin in the priesthood as toward protecting church property. There were a
number of attempts from within the catholic priesthood to reinstate
marriage as a possibility right up until the sixteenth century.

To summarize, asceticism in general and asceticism in particular
were begun as a very misguided but probably mostly sincere attempt by some
early Christians to express their spirituality. When celibacy was finally
enforced for the priesthood it was principally as a rather cynical means
of protecting church property from inheritance by the children of priests.

As a final note, consider two passages from the New Testament
which have bearing on the foolishness of asceticism as a means of
spirituality. Colossians 2:20-23 teaches that asceticism–the enforced
denial of normal (but not sinful) human pleasure has absolutely no value
in protecting us from real temptation. Also 1 Timothy 4:1-5 teaches that
those who seek to enforce asceticism (specifically celibacy, fasting from
certain foods and so forth) are from “deceitful spirits and doctrines of
demons.” Let us not fall into this temptation.

John Oakes, PhD

Let me add to this slightly in response to your question.
First of all, the whole idea of a separate priesthood in Christianity
whose function is to somehow act as a representative and go-between for
“normal” Christians to come to God is completely foreign to the
Christianity of the Bible. 1 Peter 2:4-10 describes all Christians as a
royal priesthood. The idea of a disciple of Jesus coming to God through
the mediation of a human being is anathema to biblical teaching, as seen
from 1 Timothy 2:5, “For there is one God and one mediator between God and
men, the man Jesus Christ, who gave himself as a ransom for all men….”
and other passages.

The idea of celibacy of the priesthood implies the existence of a
separate priesthood within the ranks of Christians. This concept is not
found in the New Testament. It is diametrically opposed to New Testament
teaching. For this reason, it may be an interesting historical question
where the teaching of celibacy for the priesthood of the Roman Catholic
church came from. However, for the purpose of pursuing a relationship
with God through Jesus Christ, it is not a relevant question. A better
question is whether it is a good thing for a priest of God (ie any
Christian) to be celibate. But that is another question….

John Oakes