Of course seals are serious! Did the seal on the tomb keep Jesus in there? Did the seals in Revelation keep God from His will being fulfilled? Seals are marking of someone belonging to God (in this case). But as we have plodded through already, WE can turn from God due to our free will. We can refuse God, refuse our seal, that first installment of the Holy Spirit. It seems inconceivable that someone would return to the vomit of their past life, but the reality is it HAPPENS. Despite the seal. God allows this seal to be broken, as He highly desires love from us, love which can ONLY be given freely. For love is not love if it is not free.
Regards
I have to assume from this that God allows His permanent seal to be broken, thus sending those children of God to hell, because He loves us so much. I will pray that God never loves me like that.