How can man obtain this regeneration of the spirit?

The Lord Jesus died to receive the punishment as a substitute for the sinner. The sinner—spirit, soul, and body—with all his sins, has been completely judged in the Lord Jesus on the cross. In God's sight and purpose, the death of the Lord Jesus is reckoned as the death of the people of this world. He, in His holy humanity, died for all sinful humanity. However, on man's side one work still remains; that is, by faith he must join himself—spirit, soul, and body—unto the Lord Jesus. This means that he must reckon the Lord Jesus as himself, counting the death of the Lord Jesus as his own death and the resurrection of the Lord Jesus as his own resurrection. This is the meaning of John 3:16: "That every one who believes into Him...would have eternal life." The sinner must exercise faith to believe into the Lord Jesus, to be joined unto His death, and thus be one in His resurrection. Then he will be able to obtain eternal life, which is a spiritual life (17:3), and thus he will be born anew.

We must be careful not to consider the substitutionary death of the Lord Jesus and our co-death with Him as two separate matters. Those who pay attention to knowledge and understanding have this tendency. However, it must not be so in our spiritual life. Substitutionary death and co-death should be differentiated but never separated. When one believes in the substitutionary death of the Lord Jesus, such a one has already died with Christ (Rom. 6:2). To believe that the Lord Jesus took my place of punishment is to believe that I have already been punished in the Lord Jesus. The penalty of sin is death. The punishment suffered by the Lord Jesus for us was also death; therefore, in the Lord Jesus I am already dead. Otherwise, there is no saving way. To say that He died in my place is to say that I have died in Him and have been punished in Him. (Those who trust in this fact will have this experience.)

The faith by which a sinner believes in the substitutionary death of the Lord Jesus is the believing into Christ to be joined to Him. Although many times he may only see the problems regarding the penalty of sin and not have any realization of the aspect of the power of sin, this matter of being joined to the Lord is common to every believer. One who is not joined to the Lord has not believed in the Lord and has nothing to do with Him.

Believing into the Lord in this way is to be joined to the Lord. To be joined to the Lord means to experience all that the Lord has experienced. What the Lord Jesus spoke in verses 14 and 15 of John 3 has made clear what it is to be joined to Him. It is to be joined to Him in His crucifixion and His death. Each believer in the Lord Jesus (at the least) has been united with the Lord's death positionally. But "if we have grown together with Him in the likeness of His death, indeed we will also be in the likeness of His resurrection" (Rom. 6:5). Therefore, everyone who believes in the substitutional death of the Lord Jesus has been raised up (positionally) with the Lord Jesus. Although at the time he has not yet fully experienced the meaning of the Lord Jesus' resurrection, just as he has not yet entirely experienced the meaning of the Lord Jesus' death, God has raised him up with the Lord Jesus, and in the resurrection life of the Lord Jesus he has gained a new life and is born again.