Acts 2:42 and they continued steadfastly in the Apostles Doctrine and fellowship, and in breaking of bread, and in prayers. 

Whose doctrine? The Apostles - those who were ...

• Eyewitnesses of Jesus Christ, his death, burial and resurrection
• Of the first generation to receive the understanding and experience of salvation by faith in the Life, Death, Resurrection and Eternal Name of Jesus Christ.
• Commissioned and Authorized by Jesus Christ for the preaching of THE GOSPEL

Today, we receive the Word through the scriptures, and we can receive the promise of the Holy Ghost if we will believe upon, through the Word, the message and person of the Lord Jesus Christ and obey Acts 2:38!

 The One True God

• We believe and preach the One creator God, and Father of all, who is one in being and person! 
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• Baptism in the Name of the Lord Jesus Christ is given for remission of sins. It's the teaching of the Apostles! The "promise of the Father" is fulfilled in the Baptism of the Holy Ghost (Luke 24:49. Acts 1:5,9, 2:1-4)

One God in Christ Jesus

GOD

There is only ONE GOD; the Creator of the heavens and the earth and of all mankind, manifested to mankind as FATHER (creator) SON, (saviour), and HOLY GHOST (indwelling Spirit) (Romans 8:11).

God is by nature, a spirit (John 4:24).

FATHER

God is a Spirit (John 4:24), the Eternal One, the Creator of all things; and of course, of all men, thus making Him the Father (through creation).

He is the FIRST and the LAST and beside Him there is no other. (Isaiah 44:6)
There was no God formed BEFORE Him, neither shall there be after Him.

The title "Father" used by Jesus, refers to Deity

SON

Jesus is the SON of God according to the flesh (Romans 1:3), and the very God Himself according to the Spirit. Jesus is the Christ (Matt 1:23); God made flesh (John 1:1-14); God manifested in the flesh (1 Tim 3:16); He which was, which is, and which is to come. THE ALMIGHTY (Rev 1:8; Isaiah 9:6).

To this Jesus Himself testified when he said, "He that hath seen me hath seen the father" (John 14:7-11); and "I and the Father are ONE" (John 10:30).

Since it took the shedding of blood for the remission of sins of the world (Hebrews 9:22), God as the Father being a Spirit had no blood to shed; so He prepared a body of flesh and blood (Hebrews 10:5), that He might fulfill the prophecy of Isaiah 43:11; "Beside ME there is no SAVIOUR"; and caused the angels to sing "For there is born this day in the city of David, a SAVIOUR which is CHRIST THE LORD".

Hebrews 1:3 refers to Jesus as "the express image of [God's] person". No wonder Jesus could say "he that hath seen me hath seen the Father" (John 14:9)

The title Son, used by Jesus Christ, and by Deity (such as at the Jordan baptism) refers to humanity.

HOLY GHOST

The HOLY GHOST is not the third person in the Godhead, but rather the manifestation of the Spirit of God (the Creator), and of the resurrected Christ, coming to dwell in the hearts and lives of all men who will be obedient to the gospel, and the Comforter, Sustainer, and the Keeper (John 14:16-26; Romans 8:11).

When we use the title "Holy Ghost" after the Day of Pentecost (or when Jesus used it referring ahead to this era), it is referring to the immanent God, who works in human lives for regeneration BY that spirit since Pentecost.

So there are not three GODS, or persons, but three manifestations of ONE GOD who we know by the revealed face of God, Jesus Christ, God in Flesh.

Col 2:9 For in him dwelleth all the fullness of the Godhead bodily.

"These three are one" (1 John 5:7)