The first thing we need to do to answer this question is look at the words of God in the flesh, the Word Himself, Jesus Christ the only begotten Son of God.
Jesus said that heaven and earth would pass away but His words would never pass away. He said in John 12:49-50 "For I have not spoken on My own authority; but the Father who sent Me gave Me a command, what I should say and what I should speak. And I know that His command is everlasting life. Therefore, whatever I speak, just as the Father has told Me, so I speak.”
I lay this foundation because Jesus IS the way, the truth, and the life, what He spoke IS absolute truth. What He spoke He received directly from the Father and this is what He taught His disciples and they accepted His word.
In John 3:1-7 Jesus tells the Pharisee Nicodemus a ruler of Jews what one MUST do in order to be born again. Let's take a look at what Jesus said:
"There was a man of the Pharisees named Nicodemus, a ruler of the Jews. This man came to Jesus by night and said to Him, “Rabbi, we know that You are a teacher come from God; for no one can do these signs that You do unless God is with him.” Jesus answered and said to him, “Most assuredly, I say to you, unless one is born again, he cannot see the kingdom of God.” Nicodemus said to Him, “How can a man be born when he is old? Can he enter a second time into his mother’s womb and be born?” Jesus answered, “Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, ‘You must be born again.’"
In these scriptures Jesus tells us plainly what it means to be born again. Unless we are born again we CANNOT see the kingdom of God. And unless one is born of water AND the Spirit he CANNOT enter the kingdom of God. Then He goes on to say that we MUST be born again, not should be, or need to be, or maybe, but MUST be.
Now let's look at what born of water and the Spirit is according to the scriptures. Many have a problem with the word 'water' here and say that it's the birth water, or the water of the word, but that's error and when we look at the Greek word for water in John it's the same word that's used in Matthew 3:13-17
"Then Jesus came from Galilee to John at the Jordan to be baptized by him. And John tried to prevent Him, saying, “I need to be baptized by You, and are You coming to me?” But Jesus answered and said to him, “Permit it to be so now, for thus it is fitting for us to fulfill all righteousness.” Then he allowed Him. When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, “This is My beloved Son, in whom I am well pleased.”
In these scriptures Jesus is giving us an example of what it means to be born again, He gets baptized by John and immediately He comes up from the water and John was able to see a supernatural manifestation of the Spirit of God descending from heaven LIKE a dove and landing upon Jesus. (Like the tongues of fire on the day of Pentecost, Acts 2:3)
The word 'water' here is the Greek word: Hudor
5204. hudor hoo'-dore, and genitive case, hudatos hoo'-dat-os, etc. from the base of 5205; water (as if rainy) literally or figuratively:--water. (Strong's Greek Lexicon)
This is the same word used in John 3:5
The word Spirit is also the same in Matthew and John:
4151. pneuma pnyoo'-mah from 4154; a current of air, i.e. breath (blast) or a breeze (Strong's Greek Lexicon)
Now going back to John 3:5 we can understand clearly that what Jesus said to Nicodemus is that one must be born of water (hudor-baptism) and the Spirit (pneuma-Holy Spirit)
Without these two things as Jesus said, we cannot see or enter the kingdom of God.
This is what it means to be 'born again'. Water + Spirit = Born Again!
We can read of the culmination of this teaching of Jesus in the book of Acts when the Holy Spirit was ushered in with power on the day of Pentecost and 3000 new converts to Christ heard the gospel message preached by Peter and were born of water and the Spirit.
"And being assembled together with them, He commanded them not to depart from Jerusalem, but to wait for the Promise of the Father, “which,” He said, “you have heard from Me; for John truly baptized with water, but you shall be baptized with the Holy Spirit not many days from now.”
"When the Day of Pentecost had fully come, they were all with one accord in one place. And suddenly there came a sound from heaven, as of a rushing mighty wind, and it filled the whole house where they were sitting. Then there appeared to them divided tongues, as of fire, and one sat upon each of them. And they were all filled with the Holy Spirit and began to speak with other tongues, as the Spirit gave them utterance."
The initial evidence of receiving or being filled with the Holy Spirit is speaking in other languages!
"And these signs will follow those who believe: In My name they will cast out demons; they will speak with new tongues;"
“Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.” Now when they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?” Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call.”
"And with many other words he testified and exhorted them, saying, “Be saved from this perverse generation.” Then those who gladly received his word were baptized; and that day about three thousand souls were added to them. And they continued steadfastly in the apostles’ doctrine and fellowship, in the breaking of bread, and in prayers."
In The Love Of Christ,