Your question poses an interesting connection between Coelho's work and the Old Testament in the Bible. Melchizedek first appears in the book of Genesis, the first book of the The Old Testament. He is both a king and priest, the first to be titled Kohen or Priest in the Hebrew Bible. Melchizedek means "King of Righteousness." Under the Law of Moses, only descendants of Aaron could be Kings or Priests; however, Melchizedek predates Aaron by many generations. Later, in the New Testament it is thought that he is of David's Line of priests and Kings, the same lineage of Jesus the Christ in the New Testament.
The roots of Catholic Priesthood are found in the line of Melchizedek. In the book of Genesis, Melchizedek offers a sacrifice of bread and wine. Wikipedia comments on this sacrifice in the quotation below:
Christ therefore fulfilled the prophecy of Ps 110:4, that he would be a priest "after the order of Melchizedek," at the Last Supper, when he broke and shared bread with his disciples.
Melchizedek is given various names or roles in the Old Testament. (In fact, he is given various names or role in the New Testament as well.) For example, in appearances termed "Christophanies," it is theorized that Jesus made appearances during the time of the Old Testament as Melchizedek. Another thought is that Melchizedek is actually the Archangel Michael, while others go further in the thought that Michael is actually Jesus. Both Melchizedek and Jesus. The Hebrew meaning of Melchizedek is "Prince of Peace" and "King of Salem." (Here, Salem is simply a reference to Jerusalem.)
In conclusion, it's important to note that the simple name of Melchizedek is a definite allusion to the Old Testament within Coelho's book The Alchemist. Considering that an allusion refers to an indirect reference in a work of literature, it is important to consider the use of that particular literary technique in the book especially in reference to Coelho's style.
In the Old Testament, Melchizedek is the king of Salem and a high priest who blesses Abraham after he rescues his nephew Lot from the destruction of Sodom and Gomorrah. Abraham gives him one tenth of his wealth, which is today known as tithing. Although his people practive polytheism, Melchizedek recognizes the one true God and ushers Abraham into his role as the father of the Israelites.
In The Alchemist, Melchizedek guides Santiago on his journey to achieve his dreams. In exchange for the location of the treasure in Egypt, Melchizedek demands one tenth of Santiago's sheep, demonstrating that the pursuit of riches always comes with a price and that Santiago will have to make wise choices in his dealings with other characters.