Certainty about Jesus comes from experience of Him in your life. In my podcast.
John 20:24-29 New International Version (NIV)
Jesus Appears to Thomas
24 Now Thomas (also known as Didymus), one of the Twelve, was not with the disciples when Jesus came. 25 So the other disciples told him, “We have seen the Lord!”
But he said to them, “Unless I see the nail marks in his hands and put my finger where the nails were, and put my hand into his side, I will not believe.”
26 A week later his disciples were in the house again, and Thomas was with them. Though the doors were locked, Jesus came and stood among them and said, “Peace be with you!” 27 Then he said to Thomas, “Put your finger here; see my hands. Reach out your hand and put it into my side. Stop doubting and believe.”
28 Thomas said to him, “My Lord and my God!”
29 Then Jesus told him, “Because you have seen me, you have believed; blessed are those who have not seen and yet have believed.”
But though you may not see the Lord Jesus, you can have Him with you in the person of the Holy Spirit of God.