If, by "rapture", you mean the first resurrection, then your rapture idea is not heresy.
But - - - if you mean "pre-trib rapture" . . . why do you mean this and believe this?
To me, heresy means that you prefer not only an idea but whoever is promoting that idea, in a way so you are separating yourself from Christians, in order to stay with whoever is your favorite mainly because of an idea. So, if your pre-trib rapture idea is to you only an opinion you are still testing prayerfully, then I'd say it is not heresy, in your case. Or, even if you are sure of your pre-trib rapture idea, but you are not looking down on others with a different view, I would say your idea by itself is not really a heresy. But if your pre-trib thing has you fighting with and looking down on Christians who believe in post-trib, then your belief is self-righteous and therefore heretical, self-favoring at least your own self over others.
But I would say you are not a heretic, if you favor your wrong idea but do not totally cut yourself off from other Christians with other views. After all, the disciples of Jesus struggled with one another about who was the greatest, but Jesus kept them as His disciples and did not give up on them
They were not only trying to promote some idea, but were trying to promote their own selves over others. But Jesus still did not reject them as heretics.
Even if pre-trib were correct, there can be unscriptural reasons for believing in a pre-trib rapture.
(1) Saying the Bible directly teaches pre-trib is not correct. I have found not one scripture which directly and in plain and simple wording says anything like, "Jesus will come back and resurrect us before the great tribulation."
(2) There are pre-trib people who believe certain ones will be saved during the great tribulation; yet, these pre-tribbers do not want to be here to love and help our newborn brothers and sisters who will be born during the tribulation. I find it interesting, how ones so insist we need mature seasoned pastors and other experienced church members; yet, they see no need for newborns right in the tribulation itself to have our decades-seasoned ministers and other mentors to be there for them. If the church is cleared out before the trib, which is said to be seven years long, then no Christian in the trib will have more than seven years Christian experience. So, how can a pre-trib person believe we so need our more-than-seven-years experienced ministers, but newborns in the worst of all times will not need our pastors and others???? And why don't you love them enough to volunteer to Jesus, that if He pleases you would appreciate staying here in order to help your newborn brothers and sisters in the trib? In case you would like Jesus to just whoosh you out so you don't suffer for Him and you don't care about the newborns of the trib having you to help them, then it could be your way of believing pre-trib is not only unscriptural but anti-love. Make sure about if your reasons are scriptural and according to love. "Test all things; hold fast what is good." (1 Thessalonians 5:21)
(3) Ones argue that God has not appointed us to wrath, and therefore He will not have us going through the trib. Well, what about the newborns, then, who will be in the trib.? Do you think they will not be safe from our Father's wrath? If they are safe, why wouldn't you be? Our Father is not like some abusive father who in his wrath hurts his well-behaved children along with the naughty ones. Our Father has self-control. This can be demonstrated by how Shadrach and Meshach and Abed-Nego were right in that fiery furnace and the flames did not hurt them, at all, right while those flames took out the evil men who threw them into the furnace. Like this, our Father's trib judgments will only effect evil people, and will actually help to control them so they can't mess with us except as our Father has planned and controls. If you think "God" has a self-control problem, it is Satan who has this problem, not our Father; so if you think that in the trib you would be in danger from God's wrath . . . whose child are you? And do you have God and Satan mixed up, if you feel God in His wrath would hurt you who are a His child? Satan is the one who in wrath has his self-control problem; God our Father does not have such a problem, as we also can see through Psalm 91:7 which, to me, says thousands of evil people may fall at your left and right hands but it will not come near you.
(4) Ones fear that they would not be able to make it and do well in the trib, and therefore Jesus has to rapture them before all the trouble there will be in the trib. Again . . . then . . . how is it that newborn Christians in the trib will serve the LORD (consider Revelation chapter 7), but you would not be able to do well "there"?? If you want to be raptured because you feel you don't have what it takes to serve our Father during great trouble, possibly your reason for believing in pre-trib is anti-faith. Do not doubt how our Father is able to take care of us in any situation . . . easily and breezily and beautifully in His love > depending on Him > it is not about you being able (the way you are, now) ! ! ! !
"My grace is sufficient for you, for My strength is made perfect in weakness." (in 2 Corinthians 12:9)
Grow more, discover how God proves Himself to you, then see what you believe
What you can believe can be connected with how your character is now.