"but whoever drinks of the water that I will give him will never thirst again; but the water that I will give him will become in him a well of water springing up to eternal life." (John 4:14)
Believers are never thirsty? People doubting are thirsty - they are unbelievers?
"Jesus said to them, "I am the bread of life. He who comes to me will not be hungry, and he who believes in me will never be thirsty." (John 6:35)
Believers are never hungry?
"Now our Lord Jesus Christ himself, and God our Father, who loved us and gave us eternal comfort and good hope through grace," (2 Thes 2:16)
All believers have eternal comfort and a good hope?
"By this we know that we abide in Him, and He in us, because of His Spirit He has given to us." (1 John 4:13, LITV)
All believers know they abide in Him?
"Examine yourselves, whether you are in the faith; test yourselves. Or do you not yourselves perceive that Jesus Christ is in you, unless you are disapproved?" (2 Cor 13:5, LITV)
All believers know Jesus is in them?
So believers cannot doubt their salvation, right?