Luke 3

John the Baptist not only taught the "doctrine of Christ", but he also taught many principles of the gospel as -

"...he came into all the country about Jordan, preaching the baptism of repentance for the remission of sins..."  (New Testament | Luke 3:3)

As well as -

"..I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire..."  (New Testament | Luke 3:16)

This being much of the doctrine of Christ.  And of principles of the gospel -

"...the people asked him, saying, What shall we do then?

11 He answereth and saith unto them, He that hath two coats, let him impart to him that hath none; and he that hath meat, let him do likewise.
12 Then came also publicans to be baptized, and said unto him, Master, what shall we do?
13 And he said unto them, Exact no more than that which is appointed you.
14 And the soldiers likewise demanded of him, saying, And what shall we do? And he said unto them, Do violence to no man, neither accuse any falsely; and be content with your wages."  (New Testament | Luke 3:10 - 14)

Obviously teaching humility, honesty, obedience, and caring for the poor and the needy.

Of the analogy..."baptize you with the Holy Ghost and with fire", David J. Ridges commented -

"...In the scriptures, the Holy Ghost is often compared to fire. The symbolism comes from the use of fire to purify gold. The gold ore is put in a container and fire is used to heat the container. The ore melts, the impurities float to the top, and the pure gold settles to the bottom. The impurities are then discarded and pure gold remains. Thus, the gold is purified by fire. Similarly, the Holy Ghost purifies us, if we allow it. Example: We commit sin. The Holy Ghost points it out and causes our conscience to burn within us. We respond by repenting. Thus we are purified, bit by bit."  Ridges, David J. (2010-10-01). The New Testament Made Easier, Part 1)

My thoughts are that even though fire purifies or separates the gold from the impurities, it is the atonement of Jesus Christ and His doctrine and ordinances that allow the impurities to be removed.