According to the Book of Acts, the disciples, numbering about 120, were gathered together in one accord, and in one place (2:1). When they were all filled with the Holy Spirit, they did begin to speak in "other tongues," meaning other languages (2:4-11). And what did those from the other nations hear them say? They heard them speak the wonderful works of God (2:11). I would suggest to you that what they said was much like what Jesus said to the two travelers on the road to Emmaus (Lk. 24:13-27). If they had merely spoken of Creation, the Exodus, or the Commandments, I doubt many would have gathered; they had heard all that many times before. Regardless, Peter preached the Gospel to them; he preached Christ's Death, His Burial, His Resurrection, and he included His Ascension (Acts 2:14-36; Rom. 1:16; 1 Cor. 15:1-4).
Later in the Book of Acts, Saul of Tarsus met the Lord on his way to Damascus to capture Christians (Acts 9:1-20). Luke wrote, "And straightway he preached Christ in the synagogues, that He is the Son of God (9:20).
On January 31, 1971, an atheist awoke, ate breakfast, and by noon, he had knelt in his living room and accepted Jesus Christ as his Lord and Savior. And what do you suppose was the first thing he did? With zero Bible knowledge, he began telling everyone he met that he had accepted Jesus. He told them that God had captured his attention through a "coincidence," and that after hearing the Gospel preached by the preacher God had used to get his attention, the "light bulb went on," and he believed. He believed he was a sinner (not news to him, by the way). He believed Jesus died for his sin. He believed that three days after Jesus was buried, He rose from the dead, to ever be the Lord of all who would put their trust in Him. He believed the Bible was the Word of God, and that he wanted to spend the rest of his life telling people about Him.
Now, forty-two plus years later, he still believes and serves. Because of health problems, he is limited to writing a blog about his daily Bible study. He is not famous. He is not anything special. He is just a man who fell in love forty-two years ago, and his love has never waned!
That is how you can be sure you are saved; God has become the focus of your life!