I think that it is interesting how he said that "anyone who did not receive instruction from an SSPX priest" not "a traditional Catholic priest"...

Our chapel closed down because of COVID and our pastor being old.  So, we temporarily went to the SSPX until our chapel reopened.  One of the last sermons we heard was a similar style sermon.  It was all about how everyone who went to the chapel needed to give their names and numbers to the records.  And the priest who was giving the sermon was really awkward about it as if he knew something underhanded was going on or something.  It was really weird.  My husband and I both had the same impression though we only talked about it together after Mass.  I think it is partly about the SSPX trying to be in control of people and making sure they are committed to them.

I have known people who were refused any sacraments besides Confession and Holy Communion by the SSPX if they went to other chapels and didn't fully commit to the society even if the priests knew them personally...

It left us not-at-ease because we had a baby due in a few months and the SSPX definitely knew we didn't agree with them about many things.  We were worried they might threaten not to even baptize our new baby.  So, we stopped going there as soon as we could start going to our old chapel again even though it was an hour further away.  We haven't been back since...

I learned long ago never to "sign up" for the "records".  The less the SSPX knows about you, the better.

I have had SO many run ins with nutty SSPX priests over the last 30 years...

I recall one didn't want to let a child get confirmation because the child wasn't 14 years old.  Called another sspx chapel and the priest approved confirmation even though he had never even met my child.

Another sspx priest was going to refuse First Holy Communion because my child had daily catechism instruction as part of his homeschooling rather than a once a week class after mass.  The priest refused to test my child.  We went to an independent priest 3 hours drive away instead.

A child of mine needed extreme unction recently (extremely serious auto accident) and the sspx priest refused.  Called the FSSP (1 hr from me) and they told me to call the local NO.  The local NO priest was there within an hour.  No questions asked.

Really makes you think...

This was already clearly explained to you by the first poster, but you arrogantly ignored it.  Yes, those in authority can make obedience to certain commands binding under pain or mortal sin, rendering the disobedience a grave sin.  But SSPX priests have no such authority over the faithful, so this priest cannot.

So, you are saying the mortal sin would be due to disobedience?

What "certain commands"?

I learned long ago never to "sign up" for the "records".  The less the SSPX knows about you, the better.

I have had SO many run ins with nutty SSPX priests over the last 30 years...

I recall one didn't want to let a child get confirmation because the child wasn't 14 years old.  Called another sspx chapel and the priest approved confirmation even though he had never even met my child.

Another sspx priest was going to refuse First Holy Communion because my child had daily catechism instruction as part of his homeschooling rather than a once a week class after mass.  The priest refused to test my child.  We went to an independent priest 3 hours drive away instead.

A child of mine needed extreme unction recently (extremely serious auto accident) and the sspx priest refused.  Called the FSSP (1 hr from me) and they told me to call the local NO.  The local NO priest was there within an hour.  No questions asked.

Really makes you think...

Thank you for posting this.  It definitely confirms the feelings we had that made us decide to leave the SSPX and feeling wired about it...  

Things were different until they seemed to austrocize our pastor for being too traditional, sent him away and put in a new priest.  The new one did whatever they wanted even though you could see he seemed to know he shouldn't be...  🤦

I hope your child is okay?  

How I long for the Restoration of the Church and a true good and holy Pope.  I pray every day that I may live to see such...

Short answer to the OP's question:

No.

Mortal sin is a grave offense against God. I appreciate some Trads trying to be modern-day Pharisees, adding their own man-made additions to the Divine Law, but it's no more justified than what the Pharisees did in Our Lord's time!

My question was:
"Can a priest make something which is not a sin, into a mortal sin?"

You are saying "no".

Lad is saying yes, if a priest has jurisdictional authority.

So now I am thoroughly confused.

In a way, both are true.  No, the nature of the thing itself cannot be changed by the priest, but the disobedience would be sinful (assuming he had rightful authority), so the act (or lack thereof) would become sinful (and gravely sinful) under the aspect of disobedience.

There are many things which by themselves would not be inherently sinful that become sinful under the aspect of disobedience.  So, for instance, it's not a mortal sin to eat a hamburger in and of itself.  But it would be a mortal sin to eat one on Good Friday ... due to the DISOBEDIENCE to the Church's precept.

Let's say we had a teenager.  It is no sin for him, per se, to get in a car and drive to a sporting event.  But if his father forbade it, then it would be a sin, a sin of disobedience.  This same father could say, "I forbid you to take the car to go to that event under pain of mortal sin."  If the teenager did it anyway, then he would commit a mortal sin .. of disobedience.

Normally, a parish priest would have that authority over his flock and could enjoin certain commands on his parishioiners under pain of mortal sin.  But this SSPX priest has no such authority.  He simply offers Mass, and we assist at his Mass.  That's the only relationship we have with him.  At best, he could say, "I won't admit your children to First Holy Communion unless they come to these catechism classes."  But if someone then went to a neighboring Traditional priest to receive the Sacrament, he'd be within his rights to do so and would not sin out of any kind of "disobedience" to the first priest, since he would have no authority to impose it.  SSPX like to pretend that they have authority and that people need to obey them, but it's simply not true.

My question was:
"Can a priest make something which is not a sin, into a mortal sin?"

You are saying "no".

Lad is saying yes, if a priest has jurisdictional authority.

So now I am thoroughly confused.

Maybe I can help...

There are things which are prohibited because they are evil, and things which are evil because they are prohibited.

In the latter case, the evil emanates not from magically transforming a morally good or neutral object into an evil one, but from authority which one is bound to obey.

In this case, everyone has lost sight of the fact that the priest has explicit stated he is not binding under sin (the logical conclusion being that he recognizes he can’t), but is making instruction “obligatory.”

”Obligatory” is in quotes because, once again, the priest recognizes he cannot has bilge in the strict canonical/juridical sense.

That said, he most certainly can refuse to witness the marriage if he has positive doubt regarding the ability of the parties to enter into a valid marriage (which absolutely includes being properly instructed), and this prerogative/duty has nothing to do with exercising ordinary jurisdiction.

Recall that the conciliar church denies the existence of a state of grace general spiritual necessity, and therefore considers SSPX marriages invalid per se.

Consequently, the officiating priest has a grave duty (in caritas et ex officio) to be morally certain the parties understand their marriage is certainly valid, lest they turn to conciliar tribunals once times (inevitably) get hard.

The universal trend amongst conciliar canonists and marriage tribunals since V2 has been to restrict human freedom/free will by assuming as invalidating causes for marital dissolution pop-psychological “maladies” which allegedly render consent possible (eg., “narcissism”).  

“Had only I known he was such a narcissist (or psychological liar, or whatever), I never would have consented.”  It is easy to understand, therefore, that priests must KNOW that those requesting marriage of him understand the essential criteria of marriage.

I know of a couple married for 24 years with 7 kids, married by a diocesan indult priest, who today has their marriage on the rocks because the wife stupidly believes (or has decided to make herself believe) her marriage is invalid for a whole array of petty issues.  SSPX priests have seen enough marriages fall apart when things get tough to know that instruction is essential.

And hey: If the couple doesn't like it, let them go somewhere else.

This priest is right to do what he is doing (and jurisdiction has nothing to do with it).