Even if one were to agree with your assertion, it doesn't qualify for heresy. It's not stated in Scripture "don't believe this." Pre-trib, mid-trib and post-trib rapture are all non-essential doctrine. That's my point - your assertion that belief in a pre-trib rapture is heresy is baseless. You're majoring in the minors. Be careful.
I've given you the basis for considering it heretical: the Church never believed it, it is based on a reading of Scripture contrary to that which was always believed, it is new, and it will lead those who embrace it, if they have the misfortune to live in the times of Antichrist, and do not repent of the errant belief, to damnation.
The Scriptures have no explicit prohibitions on believing that Christ has only one nature, or on separating His person into the Eternal Word of God and 'the one from the Virgin', nor on believing that Christ's human will (in the sense of capacity for willing in a human manner, not in the sense of the thing willed) was swallowed up by His divine will, yet all of these are heresies, duly condemned by councils of the Church because in one way or another they vitiate the basis of our salvation.