Alma the Elder (abt. 173B.C. - 91B.C.) was a High Priest in the land of Zarahemla. The Book of Mormon refers to him only as Alma but he is often designated as Alma the Elder to help distinguish him from his son whose name was also Alma. Alma the Elder played a significant role as recorded in the Book of Mosiah, first appearing in chapter 17. He was converted to the gospel through the preaching of Abinadi, led a secret branch of righteous families out from among a wicked people, and became high priest over the people of Nephi. He died at the age of 82 in 91 B.C (Mosiah 29:44-45).
A man named Zeniff had led a group of people out of the Nephite capital city Zarahemla in hopes of living in the land of their first inheritance, a place called, "the land of Nephi." Here they found the Lamanites living in the land, who were at the time, an enemy to the Nephites. The people of Zeniff, relative to the Lamanites, were small in numbers yet desired to live peacefully apart from the Lamanites in the same geographic area. The Lamanite king allowed this, but soon the Lamanites had waged battles against Zeniff. The people of Zeniff fought them off on more than one occasion despite their disadvantage of being small in number.
It was in this setting at the end of Zeniff's reign that Alma would have been born. He would have been raised under the rule of the idolatrous King Noah. Other than these details the Book of Mormon tells nothing specific of his early life.
Alma at a young age (around 25 years of age) found himself a Priest in King Noah's court. No indication is given as to what qualified him for (or how he obtained) this position. It appears that these priests had somewhat of a political role as well as a religious role. The priests considered themselves teachers of the law of Moses, while they also counseled with the king and passed judgment over those whom the king brought to trial.
During Alma's tenure as a priest, a Prophet came among Noah’s people proclaiming repentance, the Law of Moses, the coming Christ, and giving a warning that without repentance the people would soon come into bondage. The Prophet's name was Abinadi. The people of King Noah did not want to hear about their unrighteous behavior, so they bound him and took him before King Noah.
Noah held a council with his priests (presumably Alma was there also) as what to do with Abinadi. It was decided to question him before taking action. This was done, and Abinadi gave them a verbal lashing. Noah commanded that Abinadi be slain, but the power of God rested on Abinadi, and no one dared to touch him. Abinadi continued his speech before Noah and his priests, until he had said all that God had commanded him. Alma, being a priest and hearing all of Abinadi’s bold speech, believed what had been spoken. Alma pled with the king to let Abinadi go in peace. Noah was angry with Alma and sent men after him to kill him. King Noah also had Abinadi burned at the stake.
Alma fled and hid himself. It was during this time that Alma transcribed Abinadi’s speech. Presumably, it is this writing of Alma that gives us Chapters 13-16 of the Book of Mosiah.
Waters of Mormon
Alma began to go among king Noah’s people “privately” teaching what he had learned from Abinadi. During this time he continued to keep himself hidden from the king’s servants. “After many days there were a goodly number gathered together”. These believers gathered to a place called “The Waters of Mormon” and here Alma baptized 204 people.
The baptism of Alma is outlined in the following manner:
And when he had said these words, the Spirit of the Lord was upon him, and he said: Helam, I baptize thee, having authority from the Almighty God, as a testimony that ye have entered into a covenant to serve him until you are dead as to the mortal body; and may the Spirit of the Lord be poured out upon you; and may he grant unto you eternal life, through the redemption of Christ, whom he has prepared from the foundation of the world. And after Alma had said these words, both Alma and Helam were buried in the water; and they arose and came forth out of the water rejoicing, being filled with the Spirit. And again, Alma took another, and went forth a second time into the water, and baptized him according to the first, only he did not bury himself again in the water. 
In this scripture Alma tells us that he has been given authority to baptize but does not say where that authority has come from. It could be deduced that because he was already a priest, he had this authority passed down in a proper manner, even if Noah’s priests were corrupt. Or it could be that the authority was conferred upon Alma by an angel, similar to Joseph Smith and Oliver Cowdery’s receiving the Aaronic priesthood by the hand of John the Baptist.
Alma now found himself leading a small group of believers in their religious life, regulating their affairs, setting up days of worship, appointing other priests and overseeing a program of temporal welfare that may be seen loosely as a style of communal or consecrated living. All of these meetings were done on the outskirts of their civilization to keep them hidden from Noah. But the movement continued to grow among this small civilization and word got out. By the time “they were discovered unto the king” there were about 450 believers. 
Noah sent an army after them, but Alma and his people had been “apprised of the coming of the king’s army”  so they took their families and fled into the wilderness. They traveled eight days before settling down, and they began to build up a town and called the place Helam. The people wanted Alma to be their king, but Alma refused, citing King Noah as an example as to what Kings could do to civilizations.
Some time later, a Lamanite army that was lost in the wilderness discovered the land of Helam and took possession of it. The Lamanites set a guard over the people and left Amulon to rule over the land of Helam. Amulon had also been a priest to King Noah along with Alma and knew of Alma’s conversion, perhaps he had even hunted Alma in the past. Amulon proved to be a harsh taskmaster exceeding the authority that the Lamanites had given him by assigning heavy burdens to Alma’s people and prohibiting prayer among them. 
Given their heavy bondage, Alma led his people in an escape. They gathered their belongings throughout the night and left the following morning, the Lord causing a deep sleep to fall on Amulon and his guards. 12 days later Alma and his people arrived at Zarahemla.
Alma as High Priest
During Alma’s sojourn in the wilderness, King Noah’s regime was destroyed and his son Limhi was appointed as the people’s king. The Lamanites became more harsh in their treatment of Limhi’s people and soon the people of Limhi escaped the Lamanites and found their way back to Zarahemla.
It was not long after Limhi’s people found Zarahemla that Alma and his people also made their way to Zarahemla. Upon the return of these two sets of people, Mosiah had read to the people their record and had Alma set up churches among all the people of Nephi. He established Alma as a high priest over the church. All of Limhi’s people were baptized.
It is unclear what ecclesiastical organization was established in Zarahemla prior to Alma’s arrival, but it does appear that his arrival brought about a significant change in operation. Mosiah being a righteous king and Benjamin before him would leave one to believe that some religious organization was established, even if it was the government itself.
Alma remained high priest over the people of Nephi until his death nearly 30 years later. It was during this time that Alma’s son, Alma the Younger, persecuted the church and eventually became converted, the Lord having heard his father’s prayers for him. Alma the Younger succeeded Alma the Elder as high priest over the church.