Whoever has my commandments and keeps them, he it is who loves me. And he who loves me will be loved by my Father, and I will love him and manifest myself to him." - John 14:21
And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another. For this comes from the Lord who is the Spirit. - 2 Corinthians 3:18
Beholding Jesus is how we become like Jesus. We behold Jesus when we lovingly obey Jesus.
If we love Jesus, we will long to know Him and experience Him, especially His love, and we will strive to become like Him all of which which happens if, when and because we obey Him and thus behold Him.
So, one major and primary way we know Jesus, enjoy Jesus and become like Jesus is by lovingly obeying Jesus. And, most critically, in the context of John 14:21, Jesus is saying that if you will love each other sacrificially, it proves you love Jesus, it will result in the Father and Jesus dwelling fully in you, loving you and Jesus revealing Himself more fully to you which results in you becoming more like Jesus resulting in glory to Jesus whom you love and giving the Father glory whom you love.
"This is my commandment, that you love one another as I have loved you. - John 15:12
People who are significantly and actually being transformed into the image of Jesus are real Christians and it is very true to say that they are actually loving the brethren sacrificially because that is the door to real Christlike transformation.
There’s no Biblical way to argue against this huge principle of sanctification leading to eternal life. Yet, the church generally, has little appreciation or taste for it.
If you love Jesus, you WILL love the brethren sacrificially.
If you truly love Jesus you will truly want to see and know Jesus and experience His love for you, and thus you WILL love the brethren sacrificially.
If you love Jesus you will yearn and long to become like Jesus and you WILL love the brethren sacrificially.
If you are God’s child you will long to see Him glorified which happens when you become like Jesus which means you WILL love the brethren sacrificially.
If you want to know you have eternal life to be with the one you love, you WILL love the brethren sacrificially. Loving the brethren sacrificially is a primary proof you have eternal life, have Jesus and love Jesus, and that you are being conformed into His image.
Conformation into His image is confirmation that you actually love Jesus and have been born again.