This is where you need to read the Bible more closely. You just spoke against God’s Word.
1 John 2:3 says,
”And hereby we do know that we know him, if we keep his commandments.”
1 John 2:4 says,
”He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him.”
1 John 5:1-3
“Whosoever believeth that Jesus is the Christ is born of God: and every one that loveth him [God] that begat [who gives life] loveth him [his brother] also that is begotten of him [born of God]. By this we know that we love the children of God, when we love God, and keep his commandments. For this is the love of God, that we keep his commandments: and his commandments are not grievous.”
1 John 3:10
” In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother.”
So obviously those who believe in the name of the Son of God who have eternal life will love the brethren and do righteousness. If not, they are not of God. So it’s not just a belief alone. If it was then they could just believe in Jesus and hate their brother and not do righteousness and still have eternal life. But this is simply not the case.