I think anytime we walk in the flesh and in our own understanding, forfeiting the wisdom of God, it's sin.
Is it a sin to marry an unbeliever? Personally, I think it's very unwise. But each one of us answers to God, not man.
Not all unbelievers are non Christians.
In fact there is someone inside the Christian Church who can be considered as unbeliever.
Matthew Chapter 18
[SUP]15 [/SUP]“If your brother or sister[SUP][
b][/SUP] sins,[SUP][
c][/SUP] go and point out their fault, just between the two of you. If they listen to you, you have won them over. [SUP]
16 [/SUP]But if they will not listen, take one or two others along, so that ‘every matter may be established by the testimony of two or three witnesses.’[SUP][
d][/SUP]
[SUP]17 [/SUP]If they still refuse to listen, tell it to the church; and if they refuse to listen even to the church, treat them as you would a pagan or a tax collector.
If marrying a non Christian is wrong then never befriend any non Christian too.
Remember Love comes from friendship first.
You can marry a non Christian as long as that man does not resist your Christian faith or compel you to do bad things.
BTW some non Christians are in fact brethren of the Christians outside the Church if you'll read 1 Peter 5:9.
I don't think it is not wrong to marry these people.