People who marry unbelievers don't really believe it all themselves...they just think they do... OR...they're on the rd. to belief. The Lord will use this greatly in both lives to teach and reveal himself.
What does it mean that "they're on the rd. [road] to belief."?
Why snub them? They already know how you really feel. The minute you get all holy God will reveal some weakness in your own life. Why embarrasses yourself.
It isn't a question of "how you really feel", but rather speaking to him/her about how God 'feels' about disobedience of His perfect will, i.e., sin. Secondly, impressing upon him/her the more than likely inevitable consequences of being joined with an unbeliever (cf. 2Cor 6:14-18).
IF they are professing to believe on Christ, which also means they are allegedly disciples of Christ; those who follow Christ and all His teachings, aka: the Bible, then marrying an unbeliever is a serious sin. Those who are unrepentant of sin are to be disciplined, which is one of the three marks of the true Church.
I'm not sure what getting "all holy" means? I do know that God, according to His revealed will, again the Bible, requires that everyone be perfectly holy (Matt 5:48; Heb 13:21; 1Pet 1:15,16). So, getting "all holy" seems to be a virtue, no? Are you advocating that the brethren simply wink at this sin of marrying an unbeliever, which in the OT received serious punishment and even death by God, and take the lackadaisical attitude that everything will work out for the best?
According to the Lord Christ, we are to judge but with right judgment and with a humble attitude (Prov 27:6; Matt 7:1-5; Jam 5:19,20).