1. In v1, Jesus showed the necessity of regeneration, ‘You must be born again,’
2. In v7, and the necessity of His death to accomplish a basis for this spiritual transaction, ‘so the Son of Man must be lifted up,’
3. Then in v14, in a most solemn fashion (‘I tell you the truth,’ vv3, 5) Jesus declared that no one can ‘see,’ (v3), or enter, the kingdom of God unless he is ‘born of water’ (figure of the cleansing Word, Eph 5:26; Jas 1:18; 1 Pet 1:23;) and of the Spirit (the Holy Spirit, the Agent in regeneration), v5.
The very first word we see is “For” and it connects v. 16 to v. 15 and explains what happened to make it possible that someone can “have eternal life” (v. 15), that is, through believing in Christ.
And so, “For” or because “God so loved the world” which is an astounding statement in that context because the OT and other Jewish writings had spoken only of God’s love for his people Israel. God’s love for “the world” made it possible for “whosoever” (v. 15) believes in Christ, not Jews alone, to have eternal life.