If the heart of man is wicked, how can I protect it?

We are soul, body and spirit (our conscience). When we die "the spirit returns to God who gave it."Ecclesiastes 12:7
The body perishes but the soul is kept for judgment day.
We have a free will and we make choices, to obey the lusts of our soul or to obey the voice of our conscience. The soul that sins will die, be punished eternally, in hell.
Even though our soul is CORRUPTED we must choose against its rottenness and obey our conscience. The Holy Spirit lives in us, with our conscience, if we have accepted Jesus as Lord. still, we have the choice to obey the Holy Spirit together with our conscience or to do the
lusts of our flesh.

Is christening equal to baptism?

"But Jesus called for them, saying, "Permit the children to come to Me, and do not hinder them, for the kingdom of God belongs to such as these." Luke 18:16

A baby has no sin. Sin starts when you know right from wrong, when you can discern sin. A baby that dies already belongs to the kingdom of heaven.

There is no original sin. Sin is committed, not in inherited.

3 As I live, declares the Lord GOD, you are surely not going to use this proverb in Israel anymore. 4 Behold, all souls are Mine; the soul of the father as well as the soul of the son is Mine. The soul who sins will die.
Ezekiel 18:3-4.

"The person who sins will die. The son will not bear the punishment for the fathers iniquity, nor will the father bear the punishment for the sons iniquity; the righteousness of the righteous will be upon himself, and the wickedness of the wicked will be upon himself." Ezekiel 4:20.

Repentance and baptism is for adults who can understand that they have sinned.