Doubt & assurance

I do believe; help my unbelief!

Immediately the boy’s father cried out, “I do believe; help my unbelief!”
Mark 9:24

Now Thomas called Didymus, one of the Twelve, was not with the disciples when Jesus came. So the other disciples told him, “We have seen the Lord!” But he replied, “Unless I see the nail marks in His hands, and put my finger where the nails have been, and put my hand into His side, I will never believe.” Eight days later, His disciples were once again inside with the doors locked, and Thomas was with them. Jesus came and stood among them and said, “Peace be with you.” Then Jesus said to Thomas, “Put your finger here and look at My hands. Reach out your hand and put it into My side. Stop doubting and believe.” Thomas replied, “My Lord and my God!” Jesus said to him, “Because you have seen Me, you have believed; blessed are those who have not seen and yet have believed.”
John 20:24–29

And indeed, have mercy on those who doubt;
Jude 1:22

I have written these things to you who believe in the name of the Son of God, so that you may know that you have eternal life.
1 John 5:13

The Spirit Himself testifies with our spirit that we are God’s children.
Romans 8:16

Therefore, brothers, strive to make your calling and election sure. For if you practice these things you will never stumble,
2 Peter 1:10

My sheep listen to My voice; I know them, and they follow Me. I give them eternal life, and they will never perish. No one can snatch them out of My hand. My Father who has given them to Me is greater than all. No one can snatch them out of My Father’s hand.
John 10:27–29