The trouble is that a lot of people assume that they already are Christians. Isn’t Britain a “Christian Country”? (If that ever was true, it isn’t now!) Wasn’t I christened as a baby? Don’t I go to church? Don’t I say my prayers? Don’t I try to do my best for other people? And I certainly don’t behave like some others I could name!
If you think like that then you could be like those of whom it is written (Psalm 36:2) “In their own eyes they flatter themselves too much to detect or hate their sin”. Are you a “sinner”? Some people would be greatly offended at the suggestion. Have you ever done anything that wasn’t motivated by a desire to bring glory to God? If so, you are a sinner and “the soul that sinneth, it shall die” (Ezekiel 18:4).
So, first of all, you need to recognise your condition and ask the question, “What then must I do to be saved?”. If you don’t see yourself as you are, and don’t feel a desperate need for forgiveness in the presence of the Holy God, then you won’t ask this question.
The answer given by the apostle Peter was, “Repent and be baptised, every one of you, in the name of Jesus Christ for the forgiveness of your sins” (Acts 2:38). What exactly does that mean in practice? “Repent” means to recognise your sinful and unworthy ways, and to resolve to reject these paths for your future life. Unfortunately, you will not succeed in this endeavour however hard you try!
This is why the second part of Peter’s answer is so important. By making public confession of your commitment to Jesus, and by humbly accepting his rule over all aspects of your life, you allow him to start a process whereby he undertakes to present your soul faultless on the Day of Judgment, when “everything is uncovered and laid bare before the eyes of him to whom we must give account” (Hebrews 4:13).
In short, can you truly feel in your heart, “I know whom I have believed, and am persuaded that he is able to keep that which I’ve committed unto him against that day” (2 Timothy 1:12)? If so, then you have become a Christian.