They were filled. It is no new matter for the rage of the wicked to be inflamed when the light of the gospel is set somewhat nearer; especially when they see the increase of sound doctrine, they break out with greater violence to resist. And it is to be doubted whether Luke do mean, by the word zeal, that they were moved with a certain wicked indignation, to set themselves against Paul and Barnabas, as ambition is the mother as well of envy as of all contention; or he take zeal for indignation conceived thereupon, because they did lament that the Gentiles were made equal with the people of God. For they counted this a very unmeet thing, that the holy treasure of doctrine, which was the proper inheritance of the children, should he, as it were, at all men's feet. Gainsaying and blaspheming. They were so sore set upon resisting, that they brake out at length into blasphemy. And Satan doth for the most part drive the wicked unto such madness, that when they be overcome with reasons and discouraged, they wax harder and harder; and wittingly and willingly they spew out at length blasphemies against God and the truth. Wherefore we must take so much the more heed when as the truth of God is plainly set before us, lest, if we be carried away with a desire to speak against it, we fall straightway into that steep down. [1]

Footnotes

1 - "In illud praecipitium ruamus," we rush instantly over that precipice.

They were filled with envy - Greek: "zeal." The word here denotes "wrath in dignation," that such multitudes should be disposed to hear a message which they rejected, and which threatened to overthrow their religion.
Spake against - Opposed the doctrine that Jesus was the Messiah; that the Messiah would be humble, lowly, despised, and put to death.
Contradicting - Contradicting the apostles. This was evidently done in their presence, Acts 13:46, and would cause great tumult and disorder.
And blaspheming - See the notes on Matthew 9:3. The sense evidently is, that they reproached and vilified Jesus of Nazareth; they spake of him with contempt and scorn. To speak thus of him is denominated blasphemy, Luke 22:65. When people are enraged, they have little regard for the words which they utter, and care little how they may be regarded by God. When people attached themselves to a sect or a party, in religion or politics, and they have no good arguments to employ, they attempt to overwhelm their adversaries by bitter and reproachful words. People in the heat of strife, and in professed zeal for special doctrines, more frequently utter blasphemy than they are aware. Precious and pure doctrines are often thus vilified fled because we do not believe them; and the heart of the Saviour is pierced anew, and his cause bleeds, by the wrath and wickedness of his professed friends. Compare Acts 18:6.

The Jews - were filled with envy - See on Acts 5:17 (note). These could not bear the Gentiles, who believed in Christ, to be equal with them; and yet; according to the Gospel, it was really the case.
Contradicting - The arguments and statements brought forward by the disciples; and blaspheming, speaking impiously and injuriously of Jesus Christ. This is probably what is meant.

(18) But when the Jews saw the multitudes, they were filled with envy, and spake against those things which were spoken by Paul, contradicting and blaspheming.
(18) The favour on the very same Gospel is to the reprobate and unbelievers death, and to the elect and those who believe it is life.

But when the Jews saw the multitudes,.... The vast number of people, the inhabitants of the city, the Gentiles, that were assembled together to hear the word. This must be understood of the unbelieving Jews, who came with no good design:
they were filled with envy; at the Gentiles, that they should have the word equally preached to them, as to themselves, to whom the oracles of God were committed; or at the popularity of the apostles, that such numbers should follow them; and they might fear many converts might be made by them, from among them:
and spoke against those things which were spoken by Paul; concerning Jesus of Nazareth being the Messiah, which they denied; and concerning free justification by his righteousness, which they affirmed to be by the works of the law: and they went on
contradicting and blaspheming; publicly opposing him, and speaking evil of him, and of his doctrines; and not only so, but belching out their blasphemies against the Lord Jesus Christ. The Alexandrian copy, the Vulgate Latin, Syriac, and Ethiopic versions, leave out the word "contradicting", as redundant, it having been before said, that "they spoke against", or "contradicted" what Paul delivered; though it may not be superfluous, but express their continuance in their contradiction and opposition; and which was attended with, and issued in blasphemy: such a spirit of reproach and blasphemy is with the Jews a sign of the Messiah's coming;
"says R. Jannai (x), when you see generation after generation reproaching and blaspheming, look for the feet of the Messiah, according to Psalm 89:51.''
This they have been doing one age after another, even for many ages; and therefore, according to one of their own signs, the Messiah must be come.
(x) Shirhashirim Rabba, fol. 11. 4.

But when the Jews--those zealots of exclusive Judaism.
saw the multitudes, they were filled with envy--rather, "indignation," and broke out in their usual manner.
contradicting and blaspheming--There is nothing more awful than Jewish fury and execration of the name of Jesus of Nazareth, when thoroughly roused.

When the Jews saw the multitudes. The vast concourse, in large part Gentiles, anxious to hear of the "ensign to whom the Gentiles shall seek," aroused Jewish bigotry. Nothing ever stirred the Jews of either Palestine or of Gentile countries to such hatred as the declaration that Christ is a Savior of the Gentiles as well as the Jews. See Acts 22:21.
Contradicting and blaspheming. Contradicting the application of the prophecies to Jesus, which Paul made in his address; blaspheming by denying and speaking contemptuously of the Son of God.