Reformed theology teaches that God predestines those that are saved and those that are not saved.
Institutes, Book 3, Chapter 21, paragraph 5
This is repeated in the WCF.
What is secret is HOW or WHY God chooses each individual person.
It teaches that we don't know why, but that it's for God's own pleasure.
This would fall under Unconditional Election.
Instead Jesus came here to teach us how to be members of the Kingdom of God here on earth,
and also what we must do to be chosen/elected for eternal salvation. There are no secrets.
Here are some of Jesus' own teachings, and some not, on how to be chosen by God:
John 5:24
24“Truly, truly, I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life.
John 5:27-29
27and He gave Him authority to execute judgment, because He is the Son of Man.
28“Do not marvel at this; for an hour is coming, in which all who are in the tombs will hear His voice,
29and will come forth; those who did the good deeds to a resurrection of life, those who committed the evil deeds to a resurrection of judgment.
John 3:19, 21
19“This is the judgment, that the Light has come into the world, and men loved the darkness rather than the Light, for their deeds were evil.
20“For everyone who does evil hates the Light, and does not come to the Light for fear that his deeds will be exposed.
21“But he who practices the truth comes to the Light, so that his deeds may be manifested as having been wrought in God.”
John 6:44-45
. 44“No one can come to Me unless the Father who sent Me draws him; and I will raise him up on the last day.
45“It is written in the prophets, ‘AND THEY SHALL ALL BE TAUGHT OF GOD.’ Everyone who has heard and learned from the Father, comes to Me.
And God wishes that all be saved:
1 Timothy 2:4
4who desires all men to be saved and to come to the knowledge of the truth.
So God let's us know HOW He chooses:
Belief in God
Belief in His Son
Doing good deeds.
Be taught of God,
Learn from the Father.
Dwell with the Son.
John 15:5-6
5“I am the vine, you are the branches; he who abides in Me and I in him, he bears much fruit, for apart from Me you can do nothing.
6“If anyone does not abide in Me, he is thrown away as a branch and dries up; and they gather them, and cast them into the fire and they are burned.