The United Pentecostal Church International (UPCI) says that it has 5½ million constituents world-wide (About UPCI, upci.org).  There are more than 350,000 members in the U.S.A. and Canada combined (overview of the United Pentecostal Church International by Jack Zavada, learnreligions.com). 

History

The UPCI grew out of the Pentecostal movements in the early 20^th century.   They say, “The UPCI emerged out of the Pentecostal movement that began with a Bible school in Topeka, Kansas in 1901 and with the Azusa Street revival in Los Angeles, California in 1906.  It traces its organizational roots to 1916, when a large group of Pentecostal ministers began to unite around teaching of the oneness of God and water baptism in the name of Jesus Christ” (About The UPCI, upci.org). 

“This church was founded in 1945 by the union of the Pentecostal Assemblies of Jesus Christ and the Pentecostal Church, Inc.  Both of these bodies were formed by Oneness Pentecostals who withdrew from the Assemblies of God in 1916” (Frank Mead and Samuel Hill, Handbook of Denominations, p. 193).

Authority

They state, “The Bible is the only God-given authority which man possesses.  Therefore, all doctrine, faith, hope, and all instruction of the church must be based upon and harmonized with the Bible” (2019 UPCI Manual, Articles of Faith, Preamble).

Beliefs and Practices

1. God.  They believe that there is one person in the Godhead.  They believe that the Father, Son, and Holy Spirit are not only one being, but one person without any true distinction in person (The Oneness of God by Dr. David K. Bernard and Dr. David Norris, YouTube Channel of the UPCI).  This is sometimes referred to as modalism.  They say, “Before the incarnation this one true God manifested Himself in divers ways.  In the incarnation, He manifested Himself in the Son… As He works in the lives of believers, He manifests Himself as the Holy Spirit” (2019 Articles of Faith, The One True God).  Johnny Ramsey (Church of Christ) debated Marvin Hicks (UPCI) in Sherman, Texas December 3-7, 1973.  The proposition: “Resolved, the scriptures teach that there are three separate distinct persons in the Godhead; the Father, the Son, and the Holy Spirit.”  Affirm: Johnny Ramsey.  Deny: Marvin Hicks.
2. Salvation.  “Pardon and forgiveness of sins is obtained by genuine repentance, a confessing and forsaking sins” (Articles of Faith, Repentance).
3. Baptism.  “The scriptural mode of baptism is immersion and is only for those who have fully turned from their sins and a love of the world.  It should be administered by a duly authorized minister of the gospel, in obedience to the word of God, and in the name of our Lord Jesus Christ, according to the Acts of the Apostles 2:38; 8:16; 10:48; 19:5; and obeying and fulfilling Matthew 28:19” (Articles of Faith Baptism).  Note: They believe that the name “Jesus” alone must be used in baptism, not “the Father, the Son, and the Holy Spirit.”  They believe that the name of the Father, the Son, and the Holy Spirit is “Jesus.”  The wording matters to them (See: Jesus Only Baptism by B.H.).
4. The Lord’s Supper.  They understand the bread and the fruit of the vine “as emblems of His broken body and shed blood” (Articles of Faith, Sacrament or Communion).  I did not find anything concerning frequency of partaking.
5. Foot-washing.  Foot-washing is listed as an article of faith.  “UPCI churches practice the Lord’s Supper and foot-washing as ordinances” (Beliefs and Practices of UPCI, learnreligions.com.
6. Tithing.  “We believe tithing is God’s financial plan for His work” (Articles of Faith, Tithing).
7. Holy Spirit/Miracles.  They believe that tongue speaking continues today, and is evidence that one has received the Holy Spirit and has been saved (Is Speaking in Tongues Necessary for Salvation?  Parts 1&2, Apostolic Life in the 21^st Century, David K. Bernard YouTube).  They also believe that miraculous healing continues (Article of Faith, Divine Healing). 
8. Holiness.  They disapprove of such things as “theaters, dances, mixed bathing or swimming, women cutting their hair, makeup, any apparel that immodestly exposes the body, all worldly sports and amusement, and unwholesome radio programs and music.  Furthermore, we disapprove of any of our people having television sets in their homes (Articles of Faith, Holiness).  “We disapprove of school students attending shows, dances, dance classes, theater… wearing gymnasium clothes which immodestly expose the body… coeducation classes which involve boys and girls mixed together in swimming, calisthenics, baseball, and other mixed athletics while clothed in ungodly attire which immodestly expose the body” (Articles of Faith, Public School Activities).
9. Eternal Security.  UPCI do not believe in eternal security, or once saved always saved doctrine.  “When a person transgresses and sins against God and does not repent, he will eventually be lost and cast into the lake of fire” (Articles of Faith, The Grace of God). 

Organization

The headquarters of UPCI is located in Weldon Spring, Missouri.  Its current Superintendent is David K. Bernard (Leadership, upci.org).

“The governmental structure of UPCI is basically congregational in that the local churches are self-governing” (About UPCI, upci.org).

However, there is greater organization.  “The general organization embraces a modified Presbyterian system in that ministers meet in sectional, district, and general conference to elect officials and to conduct the business of the organization” (About UPCI, upci.org).