Quaternion, from the Latin word quaternio, means a group of four. In the ancient Roman military (see Roman Legions and Ancient Empires - Rome), a quaternion was a section, or squad, of four soldiers. When Herod Agrippa I (see The Herods) had Peter arrested for preaching the Gospel, the apostle was placed in chains and guarded by 4 quaternions, or 16 soldiers, to keep him from escaping. Unless they were aware that angels were about, the heavy guard seems unnecessary (although knowledge of the incident with Malchus may have been a factor in their handling of formerly physically-dangerous Peter) since Peter did not (after he was fully converted), nor did (almost) everyone associated with him, present any military challenge to the governing authorities. That was true for most of the early church, and most of the Saints (see also The Elect) ever since, and will be the case right up to the time of the end (the primary exception will be God's miraculously-empowered end-time Two Witnesses - any force that attempts to confine or interfere with those two Prophets during the appointed 42 months of their witness and warning to the world will experience God's charring version of "shock and awe" i.e. Revelation 11:3-6)
Peter's Miraculous Escape
Peter's arrest came at a time when the early church was facing severe and deadly persecution from Roman political authorities at the behest of unbelieving Jews i.e. nearly all of the first Christians were believing Jews who recognized Jesus Christ, also a Jew, as the long-awaited Messiah. It was a matter of Jews persecuting other Jews (see also Hate Jews?). Peter's usefulness to God was not yet complete however, so God had an angel (see What Do Angels Do?) go in and rescue him.
"Now about that time Herod the king stretched forth his hands to vex certain of the church. And he killed James the brother of John with the sword. And because he saw it pleased the Jews, he proceeded further to take Peter also. Then were the days of Unleavened Bread. And when he had apprehended him, he put him in Prison, and delivered him to four quaternions of soldiers to keep him; intending after Easter [see the Fact Finder below regarding the KJV "Easter" translation error in this verse] to bring him forth to the people."
"Peter therefore was kept in prison: but prayer was made without ceasing of the church unto God for him."
"And when Herod would have brought him forth, the same night Peter was sleeping between two soldiers, bound with two chains: and the keepers before the door kept the prison. And, behold, the angel of the Lord came upon him, and a light shined in the prison: and he smote Peter on the side, and raised him up, saying, Arise up quickly. And his chains fell off from his hands."
"And the angel said unto him, Gird thyself, and bind on thy sandals. And so he did. And he saith unto him, Cast thy garment about thee, and follow me. And he went out, and followed him; and wist not that it was true which was done by the angel; but thought he saw a vision. When they were past the first and the second ward, they came unto the iron gate that leadeth unto the city; which opened to them of his own accord: and they went out, and passed on through one street; and forthwith the angel departed from him."
"And when Peter was come to himself, he said, Now I know of a surety, that the Lord hath sent His angel, and hath delivered me out of the hand of Herod, and from all the expectation of the people of the Jews."
"And when he had considered the thing, he came to the house of Mary the mother of John, whose surname was Mark; where many were gathered together praying. And as Peter knocked at the door of the gate, a damsel came to hearken, named Rhoda. And when she knew Peter's voice, she opened not the gate for gladness, but ran in, and told how Peter stood before the gate."
"And they said unto her, Thou art mad. But she constantly affirmed that it was even so. Then said they, It is his angel."
"But Peter continued knocking: and when they had opened the door, and saw him, they were astonished. But he, beckoning unto them with the hand to hold their peace, declared unto them how the Lord had brought him out of the prison. And he said, Go show these things unto James, and to the brethren. And he departed, and went into another place." (Acts 12:1-17 KJV)
Fact Finder: Why does the King James Version of the Bible have "Easter" in Acts 12:4?
"Easter" in Acts 12:4 of the King James is an incorrect translation of the actual recorded word of the Scriptures, which was Passover (one of only a few serious translation flaws of the otherwise excellent KJV). "Easter" is no where actually found in the Book of Acts, or anywhere else in the Holy Bible. Other translations, including the New King James Version (see below), properly have Passover in Acts 12:4 i.e.
"And when he had seized him, he put him in prison, and delivered him to four squads of soldiers to guard him, intending after the Passover to bring him out to the people" (Revised Standard Version)
"And when he had taken him, he put him in prison, and delivered him to four quaternions of soldiers to guard him; intending after the Passover to bring him forth to the people" (American Standard Version)
"So when he had arrested him, he put him in prison, and delivered him to four squads of soldiers to keep him, intending to bring him before the people after Passover" (New King James Version)
"whom also having seized, he did put in prison, having delivered [him] to four quaternions of soldiers to guard him, intending after the Passover to bring him forth to the people" (Young's Literal Translation)