Some people claim that Christianity is a private matter.

However, Jesus said:

Everyone who acknowledges me before men, I also will acknowledge before my Father who is in heaven, but whoever denies me before men, I also will deny before my Father who is in heaven. -- Matthew 10:32-33 (ESV)

And John wrote:

No one who denies the Son has the Father. Whoever confesses the Son has the Father also. -- 1 John 2:23 (ESV)

We have seen and testify that the Father has sent his Son to be the Savior of the world. Whoever confesses that Jesus is the Son of God, God abides in him, and he in God. -- 1 John 4:14-15 (ESV)

Paul explains:

But what does it say?

“The word is near you, in your mouth and in your heart” (Deuteronomy 30:12)
(that is, the word of faith which we preach):

because, if you

1. confess with your mouth that Jesus is Lord and
2. believe in your heart that God raised him from the dead,

you will be saved.

For with the heart one believes and is justified, and with the mouth one confesses and is saved.

For the Scripture says, “Everyone who believes in him will not be put to shame.” (Isaiah 49:23; Joel 2:26-27)

For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him.

For “everyone who calls on the name of the Lord will be saved.” (Joel 2:32)

How then will they call on him in whom they have not believed?
And how are they to believe in him uof whom they have never heard?
And how are they to hear without someone preaching?
And how are they to preach unless they are sent?

As it is written, “How beautiful are the feet of those who preach the good news!” (Isaiah 52:7; Nahum 1:15)

So faith comes from hearing, and hearing through the word of Christ.

-- Romans 10:8-10 (ESV)

This is sometimes very hard. Even in the modern times in a "Christian Nation", some believers get persecuted when they publicly testify Jesus is Lord.