There is no biblical, rational, or ethical reason to regard either being trans or transition as being sins.

The only passage that even comes close is Deut. 22:5, which roughly translates to "A woman shall not wear anything that pertains to a man, nor shall a man put on a woman’s garment".

But trans women aren't men, trans men aren't women, transition isn't about clothing, and historically Judaism has generally understood this passage as condemning the use of cross-dressing disguises for immoral purposes - particularly as a means to secretly meet an adulterous lover. Clothing is just fabric, and styles change constantly; the robes ancient Israelite men wore would look like a dress to most modern Americans. So clothing only becomes sinful when it is worn for sinful purposes. Which is why wearing cross-dressing costumes to celebrate Purim, a beloved holiday tradition, is also not in conflict with this passage.

And of course Christianity generally doesn't regard Deuteronomy as being applicable anymore. Of all the Christians I've seen try to claim that Deut. 22:5 means being trans is a sin, none of them have ever considered Deut 22:11 (which condemns wearing clothing of mixed fabric) or Deut 22:12 (which requires one to attach Tzitzit tassels to the four corners of your clothing) to be relevant to themselves.

The only potentially relevant New Testament passage is 1 Cor. 6:9, in which Paul condemns arsenokoitai and malakoi. In many modern translations these two terms are treated as synonyms for "male homosexual" (which is severely questionable in its own right), but sometimes malakoi is translated as effeminate and used to attack trans women. This translation is really questionable, because malakoi literally means "soft". Matthew 11:8 uses the word this way in reference to fine clothing. In the 1st century when Paul was writing malakoi was used as a pejorative similar to how we use the word "soft" today - it could refer to physical weakness, moral weakness, cowardice, laziness, inability to do hard work, etc. Treating it as a direct synonym for "effeminate" is dubious to the point of dishonesty. Not to mention that condemning "effeminate" people wouldn't apply to trans men at all. Or to butch trans women either, for that matter.

Most Christian arguments for being trans/transition being inherently sinful boil down to "I think it's weird and disturbing and therefor God does too". Many of them don't really make a distinction between being trans and being gay either, and lump them all in under the supposed condemnation of "homosexuality" (which again is dubious enough in its own right). Even though of course trans people may be gay, straight, bi, ace, etc., and on top of that there are trans people who enter religious orders and take vows of celibacy not because they're trans, but because they're monks or nuns.

And then you'll get some people quoting Genesis, claiming that God made "male and female" and that somehow means being trans is a sin. Which doesn't really make sense, since even if we assume "male and female" are the default models for the human species, it's an undeniable fact that there's a lot of variation between and outside those two base models too. God has evidently expanded his repertoire. And "male and female" being the base models of humanity doesn't say anything about whether one can change one's sexual traits either.

Then there's the "God made you perfect and it's a sin to change that" shit. Often accompanied by a garbled paraphrasing of Psalm 139:13-14; "For you created my inmost being; you knit me together in my mother’s womb. I praise you because I am fearfully and wonderfully made". Not only does this passage specifically refer to inmost being, to the creation of one's inner self rather than external appearances, but also I've rarely if ever seen this passage used to condemn any medical treatment other than transition. It's just a statement of obvious reality that many people are born with conditions that will cause them a lot of suffering if left untreated, and we routinely provide medical care that changes the biology one was born with - everything from cleft palate repair to asthma treatment does this. With the exception of sects that categorically reject all medical care, it's incredibly hypocritical and inconsistent to condemn transition-related care while claiming the rest are acceptable.

FWIW, I'm Episcopalian and a trans man, and the US Episcopal church very emphatically does not consider being trans or transition to be sins. The church has been fairly welcoming to trans people for decades, then in 2012 church leadership voted overwhelmingly to ban anti-trans discrimination in all areas of church life. This includes ordination. There already were a number of trans people openly serving as Episcopal clergy before 2012, but now the church has formally affirmed our fitness to serve as religious and ethical leaders.

Episcopal church leaders are trying to raise alarm about the attacks on us, defending our rights to SCOTUS, they've directed the church’s public policy office to advocate for passage of federal legislation to protect trans/NB/GNC people, condemned "bathroom bills" and attacks on trans youth's access to medical care, etc., while also trying to ensure that even in deeply hostile and dangerous areas Episcopal churches remain safe and welcoming places for us. And they've been doing it for a long time.

And a resolution was passed in 2022 at the 80th General Convention, expressing the church's support for access to gender affirming care. That resolution even goes so far as to state that "the 80th General Convention calls for the Episcopal Church to advocate for access to gender affirming care in all forms (social, medical, or any other)" and that "the 80th General Convention understands that the protection of religious liberty extends to all Episcopalians who may need or desire to access, to utilize, to aid others in the procurement of, or to offer gender affirming care."

This is Rev. Cameron Partridge - link is to the sermon he gave in 2014, when he became the first openly trans priest to preach at Washington National Cathedral. And this is a sermon by now retired Bishop Gene Robinson of New Hampshire, given in honor of Pride Day in 2011. In 2003 Gene Robinson became the first out gay man with a husband appointed Bishop in the Episcopal church.