Christ here referred to water baptism and the renewing of the heart by the Spirit of God.
Repentance, faith, and baptism are the requisite steps in conversion.... As Christians submit to the solemn rite of baptism, He registers the vow that they make to be true to Him. This vow is their oath of allegiance.
Christ has made baptism the sign of entrance to His spiritual kingdom. He has made this a positive condition with which all must comply who wish to be acknowledged as under the authority of the Father, the Son, and the Holy Spirit. Before man can find a home in the church, before passing the threshold of God’s spiritual kingdom, he is to receive the impress of the divine name, “The Lord our Righteousness.” Jeremiah 23:6 Those who are baptized in the threefold name of the Father, the Son, and the Holy Spirit, at the very entrance of their Christian life declare publicly that they have forsaken the service of Satan and have become members of the royal family, children of the heavenly King. They have obeyed the command: “Come out from among them, and be ye separate, ... and touch not the unclean thing.” And to them is fulfilled the promise: “I will receive you, and will be a Father unto you, and ye shall be my sons and daughters.” 2 Corinthians 6:17, 18.
Henceforth the believer is to bear in mind that he is dedicated to God, to Christ, and to the Holy Spirit. He is to make all worldly considerations secondary to this new relation....
The obligations in the spiritual agreement entered into at baptism are mutual. As human beings act their part with whole-hearted obedience, they have a right to pray, “Let it be known, Lord, that Thou art God in Israel.” The fact that you have been baptized in the name of the Father, the Son, and the Holy Spirit, is an assurance that if you will claim their help, these powers will help you in every emergency. FLB 145
Then, directing that the weeping friends be sent from the room, he [Peter] kneeled down and prayed fervently to God to restore Dorcas to life and health. Turning to the body, he said, “Tabitha, arise. And she opened her eyes: and when she saw Peter, she sat up. Pr 154
Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. John 3:5 (King James Version)