That same Holy Spirit that came so powerfully on Pentecost, is the same Holy Spirit that still comes to us through Holy Baptism. His Work is ultimately to communicate Jesus to our hearts that our sins might be forgiven and that our hearts might be enlightened to know and believe all the promises of God.
On the Day of Pentecost, in Acts 2, when Peter and the apostles began speaking in tongues that day – it was clearly a known language to all the different ethnic groups that were up at Jerusalem that day. God was using all these extraordinary gifts of the Holy Spirit to help blend his Jewish/Gentile church into one body under Christ as its head.
“These men are not drunk. It’s only 9 am” … Actually, let’s come back to that in a couple seconds …
The question at hand is about certainty. When it comes to matters of faith in God our Savior, we want to be certain indeed. God is even more concerned about that. He is concerned that his love for sinners, shown so clearly in the sacrifice of his Son for us, be clearly communicated and taught to every soul in this world.