Welcome and thank you for taking a bible break to test your knowledge of God's word.
This week's question:

God told Israel through Moses;

1 Then God spoke all these words, saying,
2 "I am the LORD your God, who brought you out of the land of Egypt, out of the house of slavery.
3 "You shall have no other gods before Me.
4 "You shall not make for yourself an idol, or any likeness of what is in heaven above or on the earth beneath or in the water under the earth.
5 "You shall not worship them or serve them; for I, the LORD your God, am a jealous God, visiting the iniquity of the fathers on the children, on the third and the fourth generations of those who hate Me, (Exodus 20:1-5).

Now, it wasn't long before the Israelites made a golden calf while Moses was on Mount Sinai, and;

9 The LORD said to Moses, "I have seen this people, and behold, they are an obstinate people.
10 "Now then let Me alone, that My anger may burn against them and that I may destroy them; and I will make of you a great nation." (Exodus 32:9-10). Only with some convincing by Moses were the people spared, except three thousand men (Exodus 32:11-28). God is still a jealous God concerning idols;

18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness,
19 because that which is known about God is evident within them; for God made it evident to them.
20 For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse.
21 For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened.
22 Professing to be wise, they became fools,
23 and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures. (Romans 1:18-23).

That brings us to our question, "Who worshipped 'an unknown god'?". On Paul's second missionary journey he arrived at Athens, a city full of idols and his spirit was provoked within him because of it. He began reasoning in the synagogue with the Jews and God-fearing Gentiles and in the marketplace with those who happened to be present. Some of the Epicurean and Stoic philosophers were talking to him and said;

17 So he was reasoning in the synagogue with the Jews and the God-fearing Gentiles, and in the market place every day with those who happened to be present.
18 And also some of the Epicurean and Stoic philosophers were conversing with him. Some were saying, "What would this idle babbler wish to say?" Others, "He seems to be a proclaimer of strange deities,"--because he was preaching Jesus and the resurrection. (Acts 17:17-18). So they took Paul;

19 And they took him and brought him to the Areopagus, saying, "May we know what this new teaching is which you are proclaiming?
20 "For you are bringing some strange things to our ears; so we want to know what these things mean."
21 (Now all the Athenians and the strangers visiting there used to spend their time in nothing other than telling or hearing something new.)(Acts 17:19-21). So Paul began speaking to them in the midst of the Areopagus by using the very idols they worshipped as a positive step to teach them of the one true God that they worshipped in ignorance;

27 that they would seek God, if perhaps they might grope for Him and find Him, though He is not far from each one of us;28 for in Him we live and move and exist, as even some of your own poets have said, 'For we also are His children.'
29"Being then the children of God, we ought not to think that the Divine Nature is like gold or silver or stone, an image formed by the art and thought of man. (Acts 17:27-29).

Paul ends his lesson by saying that God has declared everywhere that men should repent because he has fixed a day for judgment, furnishing proof through Jesus by raising Him from the dead.

32 Now when they heard of the resurrection of the dead, some began to sneer, but others said, "We shall hear you again concerning this."
33 So Paul went out of their midst.
34 But some men joined him and believed, among whom also were Dionysius the Areopagite and a woman named Damaris and others with them. (Acts 17:30-34).

So then, for the answer to our question we turn and read;

22 So Paul stood in the midst of the Areopagus and said, "Men of Athens, I observe that you are very religious in all respects.
23 "For while I was passing through and examining the objects of your worship, I also found an altar with this inscription, 'TO AN UNKNOWN GOD ' Therefore what you worship in ignorance, this I proclaim to you. (Acts 17:22-23).

So there we have the answer to our question as it is the Athenians that are not only worshiping to many idols, but to make sure they are covering all their gods bases they even have an alter for an unknown god. Paul used their very idols they worshipped as a positive step to teach them of the one true God that they worshipped in ignorance! The Lord reminds us again,

14 Therefore, my beloved, flee from idolatry. (1 Corinthians 10:14),

and again;

19 We know that we are of God, and that the whole world lies in the power of the evil one.
20 And we know that the Son of God has come, and has given us understanding so that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ This is the true God and eternal life.
21 Little children, guard yourselves from idols. (1 John 5:19-21)

May we be as noble minded as some of those in Athens who had not heard the truth taught before and yet were ready to recognize it for what it really is, the power of God to save for those who act upon it, like Paul who once tried to do everything he could that was hostile to those who followed Jesus of Nazareth.