13 For this reason we also thank God without ceasing, because when you received the word of God which you heard from us, you welcomed it not as the word of men, but as it is in truth, the word of God, which also effectively works in you who believe.
14 For you, brethren, became imitators of the churches of God which are in Judea in Christ Jesus. For you also suffered the same things from your own countrymen, just as they did from the Judeans,
15 who killed both the Lord Jesus and their own prophets, and have persecuted us; and they do not please God and are contrary to all men,
16 forbidding us to speak to the Gentiles that they may be saved, so as always to fill up the measure of their sins; but wrath has come upon them to the uttermost.
for this reason (v.13) — for what he’s about to say in the rest of the verse and in the next one
without ceasing (v.13) — the same word is used in secular writings to describe an incessant cough, so Paul isn’t talking about uninterrupted thanksgiving but constantly recurring thanks
the word of God (v.13) — Paul wasn’t simply stating that the Thessalonians received his words as having come from God but also that that was truly the case.
which also effectively works (v.13) — present continuous tense — Note that while Paul was the messenger, it was the Word of God that brought about their salvation.
churches of God which are in Judea in Christ Jesus (v.14) — Galatians 1:22 — “churches of God” is always used in Scripture of the local assemblies — The churches in Judea were Jewish assemblies following the kingdom gospel preached by the twelve apostles (Acts 2:41-47), while that in Thessalonica was a Gentile church following Paul’s gospel of the mystery. But both were made up of believers in Jesus Christ, and the members of both were saved. Later, Paul called upon the Gentile churches to support the Judean churches.
your own countrymen (v.14) — The Jews in Thessalonica had stirred up the Gentile rowdies in the city to attack the believers. Both groups are probably referred to here by Paul.
persecuted (v.15) = drove us out, pursued us, chased completely out of, expelled
please (v.15) = satisfy, win the favor of — in the present continuous tense
contrary (v.15) = opposed, hostile — in Acts 27:4, it is used of the wind against a sailing vessel
forbidding us to speak (v.16) — as at Pisidian Antioch, Acts 13:45, 50; at Iconium, Acts 14:1-5; at Lystra, Acts 14:19; at Berea, Acts 17:13; and at Corinth, Acts 18:12, as well as at Thessalonica. The suggestion that God meant to send a messenger to the Gentiles aroused the Jerusalem Jews to jealous fury, Acts 22:22-23, notwithstanding the repeated testimony of the Scriptures that it was His purpose to bless all nations. — Vine, page 40.
always (v.16) — an uninterrupted succession of wrong behavior
to fill up the measure of their sins (v.16) — God permits the evil things He sees in a man, or in a nation, to grow and develop until they become manifest to other eyes than His own, that thus the righteousness of His judgments, when they do come may be put beyond dispute. So He dealt with the Amorites (Genesis 15:16), the language of which the apostle uses here from the LXX [Septuagint], and in due time judgment fell upon them (Joshua 10). Gabriel ascribed this reason for the delay of the divine retribution in Daniel 8:23. [See also Matthew 23:32] — Vine, page 41
wrath has come upon them to the uttermost(v.16) — The Jews were already a scattered people, their land under a foreign yoke, and within twenty years of the writing of these words, their temple and their city were destroyed. — Vine, page 41.