Q: I was told by a friend that the only way you were Christian was if you "asked Jesus into your heart." What is my friend talking about?
A: He is talking about having a conversion experience in which one prays to God or Jesus and asks to be forgiven of one's sins on the basis of Christ's death on the cross. The phrase "ask Jesus into your heart" is not found in the Bible, but originates in a later, Protestant evangelism campaign. It is not definitive of what a Christian is either according to the Bible or according to Church history. A person is a Christian if he is baptized and professes the Christian faith.
All Christians should take their faith seriously and devoutly cultivate his relationship with God and with Jesus, but that is not presented to us in either the Bible or the history of the Church as one of the requirements for being Christian. The New Testament regularly refers to people as Christians even though their walk with the Lord may be very shaky. Once they have been baptized, the New Testament does not deny them the title "Christian." Only by a total repudiation of the Christian faith can one lose this title.
Your friend is confusing a particular evangelistic campaign with the essence of Christianity. Periodically, to get people to take their faith seriously, evangelists have come up with questions to get people to think about their level of faith in and committment to God.
Examples of these questions are "Have you ever received Jesus into your heart?", "If God asked you why he should let you into heaven, what would you say?", "Have you made a personal committment to the Lord Jesus?", "Have you accepted Jesus as your personal Lord and Savior?", and so forth.
Questions like this are well and good--people need to be given a jolt every so often to consider whether they are living in harmony with God (St. Paul, for example, gives his Corinthian readers such a jolt when he tells them: "Examine yourselves, to see whether you are holding to your faith. Test yourselves. Do you not realize that Jesus Christ is in you? -- unless indeed you fail to meet the test!"; 2 Corinthians 13:5).
However, sometimes such evangelistic questions get repeated so often that people forget they aren't in the Bible and that Scripture does not present salvation in those terms (that is the point of coming up with the question in the first place--to phrase the idea of salvation and committment to God in a new way, a mode of expression not used in the Bible, so that people will be jolted into thinking about it).
When this happens, people end up confusing their own particular evangelistic campaign and way of phrasing things with the essence of Christianity. They then go around asking people their evangelistic question as if it the test for whether someone is a Christian, and anyone who does not give their group's formula answer is then told they are not a Christian and need to question their salvation. It sounds like this is what your friend is doing.
If you were baptized (irrespective of your age at the time) then you were by that very fact given a personal relationship with God and put into the sphere of his grace. If you are not in state of grace now it will be because you have committed a mortal sin, not because you haven't followed the particular formula of a particular evangelistic campaign.