What did the Apostles teach/preach? Did they just get up and say "Jesus died for your sins, come if you want to?"
It depends on the crowd and who is listening. As the way of the Master puts it, grace to the humble and law to the proud. Yes, we are actually told to warn them of the judgment to come
Col 1:28 whom we preach, warning every man and teaching every man in all wisdom, so that we may present every man perfect in Christ Jesus.
Col 1:29 For which I also labor, striving according to the working of Him who works in me in power.
1Co 10:11 And all these things happened to them as examples; and it is written for our warning on whom the ends of the world have come.
Luk 3:7 Then he said to the crowd that came forth to be baptized by him, O generation of vipers! Who has warned you to flee from the wrath to come?
Mat 5:21 You have heard that it was said to the ancients, "You shall not kill" --and, "Whoever shall kill shall be liable to the judgment."
Luk 10:13 Woe to you, Chorazin! Woe to you, Bethsaida! For if the mighty works which have been done in you had been done in Tyre and Sidon, they would have repented a long while ago, sitting in sackcloth and ashes.
Luk 10:14 But it shall be more tolerable for Tyre and Sidon at the Judgment than for you.
Luk 10:15 And you, Capernaum, which has been lifted up to Heaven, you shall be thrust down into hell.
Act 24:24 Then after certain days, when Felix came with his wife Drusilla, who was a Jewess, he sent for Paul and heard him concerning the faith in Christ.
Act 24:25 And as he reasoned of righteousness, temperance, and the Judgment to come, Felix trembled and answered, Go for this time, but taking time later, I will call for you.
It wasn't all Joel Osteen love talk. They warned. They reasoned
And read the letters to the churches in Revelation.