Because they believe in God and in Jesus Christ His only Son and the Holy Spirit and live their lives in accordance with Christ’s commands.
1 John 3:22
And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us.
I'll follow what the Book says; you can follow what your church teaches; and we'll BOTH be happy.
If you can show that I WON'T be happy on Judgement Day; please, Please, PLEASE!!!; show me!