of course you can have doubts. any mindful person would.
Satan always tempts everybody. christians and non christians alike.
but if you want assurance...
martyrdom is not proof of faith.
the best assurance you might have that i can think of right now is experience.
worship and emotion is one thing.
knowing for certain as much as you know any friend is something else.
pentecostals believe that the best assurance experience is the baptism of the holy spirit, when you receive the enabling power of God to go out and witness. I tend to agree with them.
Its a continual thing, and very tangible. It's not given to you during salvation itself, but upon request and God's willingness to use you. I won't even try to begin to guess His motives or what 'qualifications' he has (if any), but you can be sure that if you've been baptized in the holy spirit, and He comes upon you, and you dwell in Him and He in you, there is Never A Shadow Of A Doubt about it. You can pretend and there'd be doubts by the heap, but when it's real and you can't stop it, and you feel that fire inside that you just can't hold back, and you want to shout to the world that Jesus is Lord and He reigns and is coming... when you feel within you that deep quiet intimacy where He speaks with you and that longing for the world to turn to Him...

There can be no doubt.