The baptism of the Holy Spirit is the baptism that matters most. The physical baptism in water is just a symbol, and it is a public commitment, or promise, to devote oneself to God. But anyone, Christian or not, can be baptized; the act itself does not ensure one's salvation or even one's dedication to God. Many have been baptized to secure a job, a salary, friends, popularity, or political advantage. God reads the heart. But the baptism of the Holy Spirit is clear evidence of one's acceptance with God.
The Bible likens God's people to a bride: God's bride.
". . . and as the bridegroom rejoiceth over the bride, so shall thy
God rejoice over thee." (Isaiah 62:5)
Baptism, a ceremony by which one shows publicly a desire for lifelong commitment to God, is like a wedding ceremony. The wedding ceremony itself means little if the marriage is not consummated. The same is true with baptism: it is the Holy Spirit's abiding presence in the life that means most.
Jesus' conversation with Nicodemus helps us to understand the importance of the Holy Spirit as compared with water baptism.
"Jesus answered, Verily, verily, I say unto thee, Except a man be born
of water and of the Spirit, he cannot enter into the kingdom of God.
That which is born of the flesh is flesh; and that which is born of
the Spirit is spirit. Marvel not that I said unto thee, Ye must be
born again. The wind bloweth where it listeth, and thou hearest the
sound thereof, but canst not tell whence it cometh, and whither it
goeth: so is every one that is born of the Spirit." (John 3:5-8)
To summarize, baptism can be likened to a wedding ceremony and a marriage: the wedding is brief, but marriage is lifelong. Though they are two events, they can both be called "marriage"--one marriage. In like manner, the two baptisms are really one: they both form part of one's commitment to and relationship with Christ.