Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
Note 2 at Joh 3:3: The new birth is essential for entering into the kingdom of God (Joh 3:5). As Jesus explained to Nicodemus, this is not a second physical birth but rather a spiritual birth. Our spiritual man became dead unto (separated from) God through sin (Ro 3:23, 6:23, 7:9, 11; Eph 2:1, and 5). Just as we didn't accomplish our physical births, we cannot produce this spiritual rebirth. We are totally incapable of saving ourselves (Jer 13:23; Ro 3:10-12, 8:7-8; and Eph 2:3); therefore, we need a Savior (Tit 1:4; 2:13; 3:4, and 6). We simply believe on the Lord Jesus Christ, and we are saved (Ac 16:31). Faith is the only condition (Ro 3:28 and 10:6-9). Faith alone saves; however, saving faith is never alone. As stated in Jas 2:17-18, "faith, if it hath not works, is dead, being alone...shew me thy faith without thy works, and I will shew thee my faith by my works." Salvation is not a reformation but rather a regeneration, a new birth, a new creation (2Co 5:17), that can only be accomplished by a creative miracle of the Holy Spirit (Joh 1:13 and 3:5).
Note 3 at Joh 3:3: Throughout Jesus' earthly ministry, the Jews kept looking for Him to establish a physical kingdom here on the earth and deliver them from the oppression of the Romans (Da 7:13-14, 27; Lu 17:20; and Ac 1:6). Although at Jesus' Second Coming, the kingdom of God will physically rule over the nations of the earth (Mt 25:31-46; Re 11:15, and 20:4), Jesus' kingdom is spiritually established by His Word and not carnal weapons (2Co 10:3-5). Jesus said, "The kingdom of God cometh not with observation...behold, the kingdom of God is within you" (Lu 17:20-21). Paul said we are already in the kingdom of God (Col 1:13). The kingdom of God is therefore Christ's "invisible church," His body; it was begun during His earthly ministry and is still ruling the hearts of people today. To be a part of His church (Ro 12:5 and Eph 1:22-23), you must be born again.