The Last Chapter of the Old Testament

The Old Testament of the Christian Bible is NOT just the Hebrew Scripture. First of all, the Old Testament of the Catholic Bible is based upon the Greek translation of the Hebrew Scripture and a few books more, the Septuagint. Even after Martin Luther threw away the few Greek books to form the Protestant Old Testament, it is still different from the Hebrew Scripture because the Septuagint has a different structure.
The Hebrew Scripture consists of 3 parts: the Torah, the Prophets and the Holy Writings.
The Torah is the first five books which were supposed to be written by Moses. The books have no titles. They are identified by the first word of each book.
The Prophets consist of the Former Prophets which include Joshua, Judges, Samuel, Kings; the Latter Prophets which include the 3 major prophets (Isaiah, Jeremiah and Ezekiel) and the 12 minor prophets.
The Holy Writings consist of the Chronicles, Psalms, Job, Proverbs, Ruth, Song of Songs, Ecclesiastes, Lamentation, Esther, Daniel and Ezra/Nehemiah.
Therefore, the last book of the Hebrew Scripture is not Malachi, but Nehemiah.
The Septuagint consists of 4 parts: The Pentateuch, Historical books, Wisdom literature and the Prophets
The Pentateuch is the first five books: Genesis, Exodus, Leviticus, Numbers and Deuteronomy. These names actually came from the Septuagint.
Historical books consist of Joshua, Judges, Ruth, 1-4 Kings (i.e. 1&2 Samuel, 1&2 Kings), 1&2 Chronicles, 1&2 Esdras, Esther, Judith, Tobit, 1-4 Maccabees.
Wisdom literature consists of 151 Psalms + 14 prayers/songs, Proverbs, Ecclesiastes, Song of Songs, Job, Wisdom of Solomon, Wisdom of Sirach, Psalms of Solomon.
Prophets consists of 12 minor prophets, Isaiah, Jeremiah, Baruch, Lamentations, Letter of Jeremiah, Ezekiel, Susanna, Daniel, Bel & Dragon.
Therefore, the last book of the Septuagint is not Malachi, but Bel & Dragon.
The Catholic Old Testament basically follow the Septuagint. However it moves Judith after Tobit, Esther after Judith, 12 minor prophets after Daniel; Job before the Psalms, Lamentations before Baruch; drops 3&4 Maccabees, Psalm 151, Psalm of Solomon, appends Letter of Jeremiah to Baruch and; Susanna, Bel & Dragon to Daniel.
The Protestant Old Testament drops the 7 Greek books from the Catholic Old Testament, viz. Tobit, Judith, 1&2 Maccabees, Wisdom of Solomon, Wisdom of Sirach and Baruch.
So, the last book of the Christian Old Testament, both Catholic and Protestant, is Malachi.
The last chapter of Malachi is an appropriate link to the New Testament, which was a Christian invention.
Behold, I send my messenger to prepare the way before me, and the Lord whom you seek will suddenly come to his temple; the messenger of the covenant in whom you delight, behold, he is coming, says the LORD of hosts (Malachi 3:1).
Nobody knows when the Lord would come because His time-frame is different from ours. He would suddenly come to you and to be fair to you, He sent His messenger to prepare the way. How?
For he is like a refiner's fire and like fullers' soap;
he will sit as a refiner and purifier of silver, and he will purify the sons of Levi and refine them like gold and silver, till they present right offerings to the LORD.
Then the offering of Judah and Jerusalem will be pleasing to the LORD as in the days of old and as in former years (Malachi 3:2b-4).
Christians found that John the Baptist matched this description as a refiner, a purifier. He appeared in the wilderness to proclaim repentance and a baptism to forgive sins.
The last verses of the Old Testament are a reminder for Christians of their roots: the Commandments and statues laid down by Moses and the messages of the prophets, represented by Elijah.
"Remember the law of my servant Moses, the statutes and ordinances that I commanded him at Horeb for all Israel.
"Behold, I will send you Elijah the prophet before the great and terrible day of the LORD comes.
And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and smite the land with a curse." (Malachi 3:22-24)

In the gospel, we have already read of Gabriel's prediction on the life of John the Baptist. It was an affirmation of the prophecy of Malachi.
and he will go before him in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, to make ready for the Lord a people prepared (Luke 1:17).
When John the Baptist was born, Zechariah sang another song of praise. The following Latin version is just for reference.

benedictus Deus Israhel quia visitavit et fecit redemptionem plebi suae

et erexit cornu salutis nobis in domo David pueri sui

sicut locutus est per os sanctorum qui a saeculo sunt prophetarum eius

salutem ex inimicis nostris et de manu omnium qui oderunt nos

ad faciendam misericordiam cum patribus nostris et memorari testamenti sui sancti

iusiurandum quod iuravit ad Abraham patrem nostrum

daturum se nobis ut sine timore de manu inimicorum nostrorum liberati serviamus illi

in sanctitate et iustitia coram ipso omnibus diebus nostris

et tu puer propheta Altissimi vocaberis praeibis enim ante faciem Domini parare vias eius

ad dandam scientiam salutis plebi eius in remissionem peccatorum eorum

per viscera misericordiae Dei nostri in quibus visitavit nos oriens ex alto

inluminare his qui in tenebris et in umbra mortis sedent ad dirigendos pedes nostros in viam pacis (Luke 1:68-79)
The mission of John the Baptist was understood in a different way. He would go before the Lord. This has become a sort of consensus among the authors. Yet, what he did had been interpreted in different way. Malachi saw him as a purifier. Gabriel saw him as turning the disobedient to wisdom (i.e. fear of God). Zechariah saw his son cleaning people of their sins.
And you, child, will be called the prophet of the Most High; for you will go before the Lord to prepare his ways,
to give knowledge of salvation to his people in the forgiveness of their sins (Luke 1:76-77)
Now, which is nearest to the truth? It doesn't matter. Each of us is a baptist. Our life situations may be different. Yet, the one common core is still there ---- to prepare the way for the Lord.

Dear Lord, Your second coming is near. We are sure that Your parousia is not far away from us. May we work harder to prepare the way for Your coming among the people. Amen.