How did Paul know the “election” of the Thessalonians?

 We give thanks to God always for you all, making mention of you in our prayers, ³ remembering without ceasing your work of faith, labor of love, and patience of hope in our Lord Jesus Christ in the sight of our God and Father, ⁴ knowing, beloved brethren, your election by God. 1 Thessalonians 1:2-4

Some may see this “election” or choice as a reference to justification. Would it make sense for Paul to reference their justification here, and how did he know it?

There is reason to believe this election was with reference to them as Gentiles, and not particularly justification, since in the context, reference is made to conduct--which Paul seems to speak of approvingly.

God chose to have a people from among the Gentiles, and like Israel as a people, these Gentiles would be chosen for a purpose, to have conduct becoming the people of God.

James understood this election of the Gentiles and he quotes Scripture and makes such application during the Jerusalem council concerning the salvation of Gentiles: James answered, saying, "Men and brethren, listen to me: ¹⁴ Simon has declared how God at the first visited the Gentiles to take out of them a people for His name. ¹⁵ And with this the words of the prophets agree, just as it is written: ¹⁶ 'After this I will return And will rebuild the tabernacle of David, which has fallen down; I will rebuild its ruins, And I will set it up; ¹⁷ So that the rest of mankind may seek the LORD, Even all the Gentiles who are called by My name, Says the LORD who does all these things.' Acts 15:13-17

What God was doing was consistent with the Scripture, and it had application, though it would seem to look to the future Millennium when Israel is restored.

So, Paul was saying the gentiles were by their conduct fulfilling the election, that being as the people of God. For the people of God are to have conduct that show forth the holiness and salvation of God to those around them—just as Israel was to do as a people of God. Their character was consistent with the Divine choice of them, and in this sense Paul was “knowing” their “election.”