Jesus said we had to endure to the end.
Matthew 24:13
King James Version
13 But he that shall endure unto the end, the same shall be saved.
Keep everything in context. Jesus also said, what you do, do quickly, but each of these have a context.
This verse in 1 John 3 is explicit, no ambiguity. No other verse in Scripture will contradict it. You cannot rightfully claim that while John wrote that if you are a child of God now, that you will be like Him then, and at the same time say, that's only true if you "endure to the end".
That "enduring to the end" hasn't happened yet for either you or I, and even so, we still have His promise, "a child of God now, you shall be like Him then".
In fact the statement of enduring to the end, in it's context, is written to those who will endure the great tribulation, and that when Jesus comes, He will rescue all those who remain alive.
Scripture does not and will not contradict itself. God spoke in complete harmony.
Not everyone believes this, nor is everyone sincere in what they say. My counsel is that each of us weigh very carefully to whom we will listen, and first and foremost, to know exactly what the Scriptures state, making certain we are holding to what God says, word for word, without mentally changing the meaning in order to make it fit to our other ideas.
I can have a certain view, like that we "must endure to the end" of our lives, somehow "proving" to God that He has regenerated us, and made us His children, or however one may think of it, and then come to a verse like:
Colossians 3:1-4 KJV
1) If ye then be risen with Christ, seek those things which are above, where Christ sitteth on the right hand of God.
2) Set your affection on things above, not on things on the earth.
3) For ye are dead, and your life is hid with Christ in God.
4) When Christ, who is our life, shall appear, then shall ye also appear with him in glory.
which makes a similar kind of statement as in 1 John. Here, we read that this part applies to those who are risen with Christ (cp Romans 6), that we are dead, our life hid with Christ in God. He promise to those who are raised with Him, who are hid with Him, that when He appears, these SHALL appear with Him in glory.
We have to be able to accept these clearly stated teachings, or in this case, this is prophecy, exactly as they are written, without mentally negating any of it by adding things that are not there.
Any time we find we are not holding to the particular thing a passage states, we need to go back and find where we went wrong, so that we truly can hold all Scriptures in harmony.
Much love!