Yes, it's a sin.
Not according to what Jesus said, but according to everything that the fathers of the church have said ever since Jesus ascended to the heavens. And even if they didn't say anything about transitionign explicitly, it's really the same thing. Religious doctrine is not proscriptive like secular law, instead in religious doctrine something is forbidden if it isn't explicitly allowed (and many things are forbidden explicitly). So unless you find a very strong precedent of the fathers of the church accepting and condoning transition (which you will not) then you can be assured that it's a sin.
Of course all this depends on what church you follow. Some are more tolerant than others. Most however are not at all. And for sure it's very hard to follow Jesus' word unless you're part of the church, so if you disagree with the church your only alternative is to go off and worship on your own.
Also, I don't believe you can pick and choose only what you like from what the chruch says Jesus was about. Because religious doctrine builds on itself. If the church says that it's not ok to be gay, and you feel you disagree with them because you're gay, but that's the only thing you disagree with... tough. You can't just follow the bits you like. That's not a church, that's a mob. A really big part of being a Christian, for most denominations, is to listen to what the people say who have authority in the church. It's a flock, remember? There's shepherds. You do what they say, or you're a lost sheep, wandering the wilderness on your own.
Btw, all of the above is what I learned from being born in a christian family, and that's the reason why I left when I realised I had to transition. Because I could not find it in me to reconcile the teachings of the church with what I know about myself. And I could not accept that I could keep loving Christ, but be apart from the church- his church. I could not accept that I could be a solitary Christian, believing in my own god, and drifting further and further away from the fold. It was either sever everything, or stay (and keep from transitioning). For me there was no rhyme or reason in having my very own sollipsistic faith. So I gave up and went away.
Note that there's a ray of light: the church accepts the trans person, they don't accept the transition. You can stay and get a lot of support to be who the church wants you to be. Or you can leave and get no support to be who you want to be.
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Sorry for the mini-rant. I am a Christian and my faith hasn't waned but I found that with this condition I don't really have a problem with God as much as with other Christians.
That's what I'm saying: god is the other christians. Exactly what god are you following when it's a god only you know?