@Samuel_95 I think the key to understanding what it means to remain ‘in Christ’ is to read the entirety of John 15 and also 1 John. If you truly want to understand what John is saying, I think reading 1 John, a short but jam packed book, is the way to go. The answer is both simple and challenging - we remain in God by remaining in His love.
God is love. If we love God and neighbor we remain in Him. And how do we love? We love by first receiving the love of Christ shown through His death on the cross and then living in the power of His resurrection by walking in the Spirit. We love because He first loved us.
Hope that helps
John 15:1-4 - “I am the true vine, and my Father is the gardener. 2 He cuts off every branch in me that bears no fruit, while every branch that does bear fruit he prunes so that it will be even more fruitful. 3 You are already clean because of the word I have spoken to you. 4 Remain in me, as I also remain in you. No branch can bear fruit by itself; it must remain in the vine. Neither can you bear fruit unless you remain in me.
John 15:9-10 - “As the Father has loved me, so have I loved you. Now remain in my love. 10 If you keep my commands, you will remain in my love, just as I have kept my Father’s commands and remain in his love.
John 15:12 - My command is this: Love each other as I have loved you.
John 15:17 - This is my command: Love each other.
I John 4:12-16,19-21 - No one has ever seen God; but if we love one another, God lives in us and his love is made complete in us.
13 This is how we know that we live in him and he in us: He has given us of his Spirit. 14 And we have seen and testify that the Father has sent his Son to be the Savior of the world. 15 If anyone acknowledges that Jesus is the Son of God, God lives in them and they in God. 16 And so we know and rely on the love God has for us.
God is love. Whoever lives in love lives in God, and God in them.
We love because he first loved us. 20 Whoever claims to love God yet hates a brother or sister is a liar. For whoever does not love their brother and sister, whom they have seen, cannot love God, whom they have not seen. 21 And he has given us this command: Anyone who loves God must also love their brother and sister.