Rather, you believe it does.
because God has instituted it to be sole possessor of the truth and no other.
Rather, you believe that God has done so.
I assent to this on faith and assert it to be what God asks me to do to believe in Him.
Yes, you've got it.
If you begin with the premise that those who do not believe as you do are liars, you will get nowhere. When you begin to see that others believe what they believe on faith, and assert it to be what they believe God asks of them... then you can begin dialogue.
Hence the Sola Scriptura group, which is Protestantism itself, has taken the truth that the Bible is true to the Gospel of the Apsotles and over emphasized it with the loss of the truth that the Cannon of the Bible was decided in order to be in tune with the teaching of the Church. It was decided in 382 at the Council of Rome.
Or the truth that God is omnipotent is taken by some to negate man's free will; ignoring the fact that God created Adam and Eve with free will. Therefore free will for man is part of God's plan.
So a heresy is a truth that has been exagerated at the expense of the rest of the Truth.
Is it a lie if I tell you that I saw an accident and the driver of one of the cars ran off after the accident and I don't tell you that the 2nd driver was shooting a gun at him, even though it was plainly obvious? That is heresy.
When Saul was riding to Demascus to kill Christians there he was knocked off his horse and Jesus asked him, "Why do you persecute ME?" The Holy Catholic Church is Jesus, the body of Christ. He gave his promise to the Apostles to be with them until the end of time. It ain't over just yet.