The 'forgotten' Trinity verses.

All too often, defending the Trinity from the Bible comes down to merely looking to a few verses that show the divinity of Jesus or the Holy Spirit. This is the approach many apologetics sources today take. But this approach leaves out an equally important and traditional approach, which is to look to the texts which speak of all three Persons within the same breath. What is important to keep in mind when reading these texts is to see them as the early Christians, especially Jewish Christians saw them, namely by recognizing the simple fact that no created thing can or should be mentioned alongside God, since this implied the blasphemy of making created things equal to Him. Another thing to keep in mind is that when the term "God" is used in the Bible, it quite often refers to the Father specifically, and as a way to distinguish between the Son and Holy Spirit. There are other passages that mention the Trinity which are good to know about, but here are some of the more prominent ones:

• Matthew 3:16 After being baptized, Jesus came up immediately from the water; and behold, the heavens were opened, and he saw the Spirit of God [the Father] descending as a dove and lighting on Him, 17 and behold, a voice from heaven said, “This is my beloved Son, with whom I am well pleased.”
  
  
• Matthew 12:28 But if I [Jesus] cast out demons by the Spirit of God [the Father], then the kingdom of God has come upon you.
  
  
• Matthew 28:19 Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit . . .
  
  
• Luke 3:22 And the Holy Spirit descended upon Him [Jesus] in bodily form like a dove, and a voice came out of heaven, “You are My [the Father’s] beloved Son, in You I am well-pleased.”
  
  
• John 14:26 But the Helper, the Holy Spirit, whom the Father will send in My [Jesus’] name, He will teach you all things, and bring to your remembrance all that I said to you.”
  
  
• John 15:26 When the Helper comes, whom I [Jesus] will send to you from the Father, that is the Spirit of truth who proceeds from the Father, He will testify about Me . . .
  
  
• Acts 1:4 Gathering them together, He [Jesus] commanded them not to leave Jerusalem, but to wait for what the Father had promised, “Which,” He said, “you heard of from Me 5 for John baptized with water, but you will be baptized with the Holy Spirit not many days from now.”
  
  
• Acts 2:33 Therefore having been exalted to the right hand of God [the Father], and having received from the Father the promise of the Holy Spirit, He [Jesus] has poured forth this which you both see and hear.
  
  
• Acts 10:38 You know of Jesus of Nazareth, how God [the Father] anointed Him with the Holy Spirit and with power, and how He went about doing good and healing all who were oppressed by the devil, for God was with Him.
  
  
• Romans 1:4 Who was declared the Son of God [the Father] with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord . . .
  
  

Overview of the Old Testament Priesthood

The following quotes are taken from the Catholic Encyclopedia entry on "Priesthood," which I think are worth sharing. I will quote the article freely, with slight edits for readability and keeping things brief.

The word priest is derived from the Greek presbyteros (the elder). By the term is meant a (male) person called to the immediate service of the Deity and authorized to hold public worship, especially to offer sacrifice.

In the age of the Patriarchs the offering of sacrifices was the function of the father or head of the family (cf. Genesis 8:20; 12:7, etc.; Job 1:5). Hummelauer's hypothesis that this pre-Mosaic priesthood was abolished in punishment of the worship of the golden calf (Exodus 32) can hardly be scientifically established.

In the Mosaic priesthood we must distinguish: priests, Levites, and high-priest.

Priests. It was only after the Sinaitical legislation that the Israelitic priesthood became a special class in the community. From the tribe of Levi, Jahweh chose the house of Aaron to discharge permanently and exclusively all the religious functions; Aaron himself and later the first-born of his family was to stand at the head of this priesthood as high-priest, while the other Levites were to act, not as priests, but as assistants and servants. The solemn consecration of the Aaronites to the priesthood took place at the same time as the anointing of Aaron as high-priest (Exodus 29:1-37; 40:12 sqq.; Leviticus 8:1-36). 

The official duties of the priests related partly to their main occupations, and partly to subsidiary services. To the former category belonged all functions connected with the public worship, e.g. the offering of incense twice daily (Exodus 30:7), the weekly renewal of the loaves of proposition on the golden table (Leviticus 24:9), the daily offering of the morning and evening sacrifices, especially of the lambs (Exodus 29:38 sqq.). As subsidiary services the priests had to sound the trumpets announcing the holy-days (Numbers 10:1 sqq.), declare the lepers clean or unclean (Leviticus 13-14; Deuteronomy 24:8; cf. Matthew 8:4).