A lie can be a mortal or a venial sin. Scripture says God is impartial. As such, since they got capital punishment, their sin ''has'' to have been a mortal sin. Their ''lie'' therefore has to have been a devastating one. We can see it was, when we read the context of it all.
It does not help zooming in on one scripture and teaching that as the A-Z on a matter when the message is illogical and defies the rest of scripture. That is called cherry picking. Half truths. Which are probably worse then not quoting scripture and giving opinions.
		
		
	 
If a Christian can commit a "mortal sin" as you call it.  Then they can lose their salvation.
You have been given much more than one scripture about this.
John 6:66;  
As a result of this many of His disciples withdrew and were not walking with Him anymore.
John 6:67;  So Jesus said to the twelve, "
You do not want to go away also, do you?"
2Tim 2:17;  and their talk will spread like gangrene. Among them are Hymenaeus and Philetus,
2Tim 2:18;  men who have 
gone astray from the truth saying that the resurrection has already taken place, and they 
overturn the faith of some.
Matt 24:10;  "At that time 
many will fall away and will betray one another and hate one another.
1Tim 4:1;  But the Spirit explicitly says that in later times 
some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons,
Phm 1:24;  as do Mark, Aristarchus, Demas, Luke, my fellow workers.
2Tim 4:10;  for Demas, having loved this present world, has deserted me and gone to Thessalonica; Crescens has gone to Galatia, Titus to Dalmatia.
Demas was a "fellow worker" in the church.  (a believer)  who left Paul because he "loved this present world".
We know what the Bible says about loving the world.
Luke 8:13;  "Those on the rocky soil are those who, when they hear, receive the word with joy; and these have no firm root; they 
believe for a while, and in time of temptation fall away.
2Pet 2:20;  For if, after they have 
escaped the defilements of the world by the knowledge of the 
Lord and Savior Jesus Christ, they are 
again entangled in them and are overcome, the last state has become worse for them than the first.
2Pet 2:21;  For it would be better for them not to have known the way of righteousness, than having known it, to 
turn away from the holy commandment handed on to them.
2Pet 2:22;  It has happened to them according to the true proverb, "A DOG RETURNS TO ITS OWN VOMIT," and, "A sow, 
after washing, returns to wallowing in the mire."
After you've been given 5 or 6 examples... it's no longer cherry picking.
Rev 2:4;  'But I have this against you, that you have left your first love.
Rev 2:5;  'Therefore 
remember from where you have fallen, and repent and do the deeds you did at first; or else I am coming to you and will remove your lampstand out of its place—
unless you repent.