“as a symbol of your repentance.” The Greek can be expressed that way, even if it is not the predominant way eis and the accusative is translated. D. A. Carson expresses the situation well:
The phrase “for repentance” (eis metanoian) is difficult: eis plus the accusative frequently suggests purpose (“I baptize you in order that you will repent”). Contextually (v. 6), this is unlikely, even in the telic sense suggested by Broadus: “I baptize you with a view to continued repentance.” But causal eis, or something very close to it, is not unknown in the NT (cf. Turner, Syntax, pp. 266-67): “I baptize you because of your repentance.” (The Expositor’s Bible Commentary; F. Gaebelein, general editor).
We assert the scope of Scripture shows that John did baptize people because of their repentance, and indeed, it was because of their repentance that the people came to John to be baptized by him. Indeed, Luke 7:29-30 make it clear that the sinners came to John to be baptized while the religious leaders rejected God’s purpose for themselves by not allowing themselves to be baptized by John, which would have involved their publicly confessing their sins.
The water baptism was a symbol, an outward demonstration, of the inward repentance that had happened in the heart of the people who came to be baptized. Many scholars and translators recognize this, and Daniel Wallace expresses it well: “Water baptism is not a cause of salvation, but a picture; and as such it serves both as a public acknowledgment (by those present) and a public confession (by the convert)....” (Greek Grammar Beyond the Basics).
Scholars and translators express how water baptism is a symbol or picture of the inner work of God in different ways in their writings. For example, Charles Williams translates the Matthew 3:11: “I am baptizing you in water to picture your repentance” (The NT in the Language of the People). Ann Nyland has: “I baptize you in water to show that you have changed your minds” (The Source NT). J. B. Phillips says: “I baptize you with water as a sign of your repentance” (NT in Modern English). Goodspeed’s New Testament reads: “I am baptizing you in water in token of your repentance.” Davies and Allison, after examining other possible interpretations, conclude: “It is, however, better to endorse a more nuanced position: baptism presupposes and expresses repentance” (International Critical Commentary: Matthew). Robert Mounce (New International Biblical Commentary) writes as if John is speaking in the first person: “‘My baptism’ he might say, ‘indicates you have repented.’” See commentary on Mark 1:4.
“with holy spirit or with fire.” This is the gift of holy spirit. The Messiah will baptize every person with either the gift of holy spirit or the fire of God’s judgment. [For more information on “holy spirit or fire,” see commentary on Luke 3:16. For more information on the uses of “holy spirit,” see Appendix 6: Usages of ‘Spirit’].