If you define a 'true Christian' as someone who will receive salvation, then I suspect you will be waiting a while for an answer - because the Bible categorically states both.
FAITH ALONE
A man is justified by faith without the deeds of the law. -- Romans 3:28
A man is not justified by the works of the law, but by the faith of Jesus Christ. -- Galatians 2:16
The LORD shall ... save them, because they trust in him. -- Psalm 37:40
Whosoever believeth in him should not perish, but have everlasting life. -- John 3:16
Verily, verily, I say unto you, He that believeth on me hath everlasting life. -- John 6:47
Believe on the Lord Jesus Christ, and thou shalt be saved. -- Acts 16:31
DEEDS
Faith, if it hath not works, is dead, being alone. -- James 2:17
And shall come forth; they that have done good, unto the resurrection of life. -- John 5:29
For the Son of Man will come in the glory of His Father with His angels, and then He will reward each according to his works. -- Matthew 16:27
If you want to enter into life, keep the commandments. He saith unto him, Which? Jesus said, Thou shalt do no murder, Thou shalt not commit adultery, Thou shalt not steal, Thou shalt not bear false witness, Honour thy father and thy mother: and, Thou shalt love thy neighbour as thyself. -- Matthew 19:17-19
Blessed are they that do his commandments, that they may have right to the tree of life. -- Revelation 22:14
OR MAYBE YOU NEED TO BE BAPTISED TOO
He that believeth and is baptized shall be saved. -- Mark 16:16
Baptism doth also now save us (not the putting away of the filth of the flesh). -- 1 Peter 3:21
OR MAYBE, JUST ASK
Whosoever shall call on the name of the Lord shall be saved. -- Acts 2:21, Romans 10:13
As for me, I will call upon God; and the LORD shall save me. -- Psalm 55:16
Ask, and it shall be given you; seek, and ye shall find; knock, and it shall be opened unto you: For every one that asketh receiveth; and he that seeketh findeth; and to him that knocketh it shall be opened. -- Matthew 7:7-8