Lexicon

G3316 - mesites

Strong's: G3316 · BLB lookup Pronunciation: mes-ee'-tace Part of speech: masculine noun Root: from mesos (μέσος, G3319), "middle" NT occurrences: six: Galatians 3:19, 20; 1 Timothy 2:5; Hebrews 8:6; Hebrews 9:15; Hebrews 12:24

Christ as "King of kings and Great High Priest," the mediator of a better covenant (Hebrews 8:6). Icon by Michael Damaskinos, second half of the 16th century, Byzantine and Christian Museum, Athens. Public domain, via Wikimedia Commons.

Semantic range (Thayer / BDAG)

One who stands in the middle between two parties, a go-between who brings estranged parties to agreement: mediator, arbiter, negotiator.
A guarantor or surety of an agreement, the one who stands behind its terms.

In ordinary Hellenistic usage the word belonged to law and commerce: the arbiter in a dispute, the broker of a contract, the person who vouched for an agreement. The Septuagint uses it at Job 9:33, where Job laments that there is no "umpire" (ASV) between himself and God who could lay a hand on both.

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Theological force

In the New Testament mesitēs is a covenant word. Every use concerns the relation between God and His people under a covenant, and four of the six name Jesus Christ.

The Mosaic covenant. The law "was ordained through angels by the hand of a mediator" (Galatians 3:19, ASV), usually understood as Moses. Paul's next sentence, "a mediator is not a mediator of one; but God is one" (Galatians 3:20, ASV), is famously difficult; its general thrust is that the law came through an intermediary between two parties, while the promise to Abraham came from God directly.
The one mediator. "For there is one God, one mediator also between God and men, himself man, Christ Jesus, who gave himself a ransom for all" (1 Timothy 2:5-6, ASV). The two parallel uses of heis ("one") match the oneness of the mediator to the oneness of God, and the anarthrous anthrōpos stresses that the mediator is Himself human. Verse 6 defines His mediation by His self-giving as a ransom (antilytron; see G3083 - lytron).
The mediator of a better covenant. Christ "is also the mediator of a better covenant, which hath been enacted upon better promises" (Hebrews 8:6, ASV); "for this cause he is the mediator of a new covenant, that a death having taken place for the redemption of the transgressions that were under the first covenant, they that have been called may receive the promise of the eternal inheritance" (Hebrews 9:15, ASV); believers have come "to Jesus the mediator of a new covenant, and to the blood of sprinkling" (Hebrews 12:24, ASV). In Hebrews the mediator is also the high priest and the sacrifice. See New Covenant and G1242 - diatheke.

Mediator and intercessor are different words. The New Testament's intercession vocabulary is entynchanō ("to intercede," Romans 8:27, 8:34; Hebrews 7:25), hyperentynchanō ("to intercede on behalf of," Romans 8:26) and the noun enteuxis ("intercession," 1 Timothy 2:1). Mesitēs is never used of believers praying for one another. In 1 Timothy 2 the two vocabularies sit four verses apart: believers are told to offer enteuxeis for all people, because there is one mesitēs. The passage itself distinguishes interceding from mediating. See Intercession of the Saints.

One, not only. Moses is called a mediator in Galatians 3:19, so the noun is not reserved for Christ. What is unique in 1 Timothy 2:5 is the scope ("between God and men," not between God and one nation) and the ground (a ransom "for all"). That role belongs to Christ alone.

Notable verses

Galatians 3:19-20, the law "ordained through angels by the hand of a mediator"
1 Timothy 2:5-6, "one mediator also between God and men, himself man, Christ Jesus"
Hebrews 8:6, "the mediator of a better covenant"
Hebrews 9:15, "the mediator of a new covenant, that a death having taken place"
Hebrews 12:24, "Jesus the mediator of a new covenant, and... the blood of sprinkling"