Power -

to go into all the world,
to preach to the whole of creation,
to make disciples of all nations;
 

Power -

to witness,
to proclaim who Jesus is,
to declare what God has done in and through Him;
 

Power -

to tell the good news (i.e. the Gospel),
to announce the need of repentance and
to tell of forgiveness of sins;
 

Power -

to bring men and women into obedience to Christ.

     (i) 

The disciples, once cowardly, fearful, anxious, doubtful – Peter, Thomas, and the rest - boldly confessed their faith in Jesus Christ.
 

     (ii) 

The disciples were telling, speaking, talking, "nineteen to the dozen".

Communicating the Gospel to unbelievers – "in our own languages we hear them speaking about God's deeds of Power"; (Acts 2:6-8) – and Peter preached openly, fearlessly, with fluency, conviction, authority!
 

    (iii) 

And men and women responded, and not in ones and twos, but hundreds and thousands - 3000 were baptised!

And this did not happen once and once only, it went on happening.  The disciples were arrested, imprisoned, punished, forbidden to speak.  It made no difference.  "We must obey God rather than man", (Acts 5:29)

(See also Acts 4:20 and 4:31)

We can repent -

think again, change our ways, turn from our sins;
 

We can pray -

ask, seek, knock in humility, in all sincerity - for the heavenly Father will give the Holy Spirit to those who ask him,
(Luke 11:9-13);
 

We can be obedient -

"a new commandment I give to you, that you love one another...." (John 13:34-35) - "if you love me, you will keep my commandment", (John 14:15).