I've read articles which interpret the word translated as "spirit" in Luke 1:17 to mean something like "motivational attitude" as in "team spirit." I don't think that is what Luke 1:17 means. Let's compare the language of Luke 1:17 with other scripture:
Luke 1:17 - [Angelic messenger speaking to Zacharias] - And he (Gk. autos) shall be coming first in view of him (Gk. autou) in the spirit (Gk. pneumatos) and ability of Elijah...
James 2:26 - As even the body apart from the spirit (pneumatos) dead is ...
To James, the word pneumatos meant the spiritual force which gives life to the material body. It seems likely that John used the word in the same sense. Therefore, "he" is going to be born with the spirit that gave life to Elijah's body giving life to his body.
It is possible, though, to question the antecedent of the pronoun autou ("him") from Luke 1:17. This might have referred to either John or perhaps Jesus (for whom it could be said that John "came first in the sight of").
Returning to your question around John 1:19-21, let's extend our research three more verses, to the end of the conversation in question, that is through John 1:24.
First, John said to the priests in 1:23 that John is the one sent to make the way straight for the Master, as per the prophecy from Isaiah.
Most important however, is John 1:24: "And the ones having been dispatched were out of the Pharisees."
Compare this to Jesus' teaching as to what one does with a Christian brother who will not repent from sin against you even when your church has asked them to, Matthew 18:17, "... let him be as unto you as one of those from nations (Gk. ethnikos) and the tax-collector."
A Pharisee is one of those from nations. The relevance of John 1:24 may be that John's statements to his inquisitors were less than forthcoming. It is possible that his answer "I am not" might have been further elaborated, "I am not, I am his return, named John."
In other words John 1:19-24 is the presumably true account of John's possibly misleading responses to hostile inquisitors.