Dr. Stephen J Nichols provided a 2 minute biblical answer to the above question. I think that the above question occurs to every Christian at some point; possibly frequently.
Dr. Nichols explained why Christians have such recurring doubts, and he described what makes assurance wax and wane.
The above 2 minute video is an answer to a question provided to #AskLigonier. Check out Ligonier Ministries on YouTube for sermons, and Christin teaching; furthermore, if you liked the above answer, then you might want to explore their ministry at Ligonier.org. Many biblical pastors are affiliated with Ligonier. They seemingly address all areas of the Christian life from the Bible.