1. When we believe that Jesus is the Christ, we open our mouths. (Ezk 29:21)
    Ezk 24:25-27, Joel 2:28, Acts 2:1-18, Acts 4:18-20

In the Old Testament, God spoke of raising up Christ from the descendants of David on that day, and through Him who will come, all will meet God. (Ezekiel 29:21, Ezekiel 24:25-27)

Ezekiel 29:21 In that day will I cause the horn of the house of Israel to bud forth, and I will give thee the opening of the mouth in the midst of them; and they shall know that I am the Lord.

Ezekiel 24:25 Also, thou son of man, shall it not be in the day when I take from them their strength, the joy of their glory, the desire of their eyes, and that whereupon they set their minds, their sons and their daughters,
26 That he that escapeth in that day shall come unto thee, to cause thee to hear it with thine ears?
27 In that day shall thy mouth be opened to him which is escaped, and thou shalt speak, and be no more dumb: and thou shalt be a sign unto them; and they shall know that I am the Lord.

God said that He would pour out His Spirit on those who believed in Him. (Joel 2:28)

Joel 2:28 And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions:

God poured out the Holy Spirit on all who believe in Jesus as the Christ to open their mouths to preach the gospel. (Acts 2:1-18)

Acts 2:1 And when the day of Pentecost was fully come, they were all with one accord in one place.
2 And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.
3 And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.
4 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
5 And there were dwelling at Jerusalem Jews, devout men, out of every nation under heaven.
6 Now when this was noised abroad, the multitude came together, and were confounded, because that every man heard them speak in his own language.
7 And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak Galilaeans?
8 And how hear we every man in our own tongue, wherein we were born?
9 Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judaea, and Cappadocia, in Pontus, and Asia,
10 Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes,
11 Cretes and Arabians, we do hear them speak in our tongues the wonderful works of God.
12 And they were all amazed, and were in doubt, saying one to another, What meaneth this?
13 Others mocking said, These men are full of new wine.
14 But Peter, standing up with the eleven, lifted up his voice, and said unto them, Ye men of Judaea, and all ye that dwell at Jerusalem, be this known unto you, and hearken to my words:
15 For these are not drunken, as ye suppose, seeing it is but the third hour of the day.
16 But this is that which was spoken by the prophet Joel;
17 And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams:
18 And on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy:

We cannot but say that Jesus is the Christ. (Acts 4:18-20)

Acts 4:18 And they called them, and commanded them not to speak at all nor teach in the name of Jesus.
19 But Peter and John answered and said unto them, Whether it be right in the sight of God to hearken unto you more than unto God, judge ye.
20 For we cannot but speak the things which we have seen and heard.