In John 3:16, Jesus says that God so loved the world.

He didn't say, God so loved Israel, and he didn't say God so loved the Jews, and he didn't say, God so loved the elect.

But the world:

John 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

This is not speaking of during the thousand year kingdom, as then, in addition to believing in Jesus, it will also involve submitting to him as King.

It is also not speaking of during the tribulation, even though people will be saved, then, it will be a time for the seals, trumpets and vials upon the world.

But this appears to speaking of the church age, when, beginning with the gospel of grace revealed to Paul, the gospel went also to Gentiles all over the world.

So, when Jesus said, For God so loved the world, it looks to be prophetic of the mystery, which he would later reveal through Paul.

II Corinthians 5:19 To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation.

And John 1:29 also looks to be a fulfillment of that mystery nugget in John 3:16, for God so loved the world:

John 1:29 The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which taketh away the sin of the world.

And also in John's epistle, it speaks of beyond the children of Israel, but the whole world:

I John 2:2 And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world.

And the Samaritans said to the woman, that Jesus is the Christ, the Saviour of the world:

John 4:42 And said unto the woman, Now we believe, not because of thy saying: for we have heard him ourselves, and know that this is indeed the Christ, the Saviour of the world.

Before ascending to heaven, Jesus told his disciples to preach the gospel to every creature in all the world:

Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.

The gospel of the kingdom would be preached initially, and will again be preached during the tribulation.

But the mystery, of the gospel of grace, appears hidden in Mark 16:15, until Jesus revealed that gospel to Paul.

The gospel of grace indeed has gone into all the world, over the past couple millennia:

Romans 10:18 But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world.

Which Paul referred to:

Colossians 1:6 Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth:

  23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;

And that revelation of the mystery gospel of grace was made manifest by the preaching of Paul, and by the scriptures of the old testament prophets:

Romans 16:25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,

  26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith:

John 3:16 was said to Nicodemus, before the cross, before the resurrection, before pentecost, and before the gospel of grace was revealed to Paul.

However, it seems that there was a nugget of the mystery, hidden in there, that could later be known, by comparing it to the things that Paul said.

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