"I have put these things in writing for you who have faith in the name of the Son of God, so that you may be certain that you have eternal life." (1 John 5:13 bbe)
The great news of the Gospel is that we can know for sure that we are saved. And that once we are saved we will be kept by the power of God forever. We are now a son (or daughter) of God and subject to His chastening, but he will never let us go. We are assured of going to heaven. Therefore we are free. Free to serve God with a pure motive -- not to gain salvation, but in thanks and adoration because we already have it. It was a free gift and He will never take it away.
Dear friend, if you are not sure that you are saved, seek this above all else. You can know, and you should know. It is this experience that fully qualifies you to be a witness for Jesus Christ.
If you have doubts, see the Are You Sure tract.