You Can Be Sure

How can you be sure that you are saved? According to Apostle John, you just have to believe.
For God loved the world in this way: He gave his one and only Son, so that everyone who believes in him will not perish but have eternal life. (John 3:16 CSB)
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (John 3:16 KJV)
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him. (John 3:36 ESV)
Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life. (John 5:24 ESV)
Truly, truly, I say to you, whoever believes has eternal life. (John 6:47 ESV)

So do you believe? Can you take God's word at face value? If you can, you are saved.
Jesus said to her, "I am the resurrection and the life. Whoever believes in me, though he die, yet shall he live, and everyone who lives and believes in me shall never die. Do you believe this?" (John 11:25-26 ESV)
All who believe in the Son of God know in their hearts that this testimony is true. Those who don't believe this are actually calling God a liar because they don't believe what God has testified about his Son. And this is what God has testified: He has given us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have God's Son does not have life. (1 John 5:10-12 NLT)

But will I lose my salvation? No. No one can lose their salvation.
And this is the will of him who sent me, that I should lose nothing of all that he has given me, but raise it up on the last day. (John 6:39 ESV)
I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father's hand. I and the Father are one." (John 10:28-30 ESV)

But there are Christians who became non-Christians.
No. They were not Christians from the beginning.
They went out from us, but they were not of us; for if they had been of us, they would have continued with us. But they went out, that it might become plain that they all are not of us. (1 John 2:19 ESV)