It is possible for the police or law enforcement to spy on your text messages by contacting the cellphone company and getting records of those text messages without a warrant.
But if they haven't gotten a warrant to do so then it likely wouldn't hold up in court because the warrant is used to protect your rights.
That's like the police going through your home and belongings without a warrant and finding something illegal and then taking you to court for it.
If the police didn't have a warrant to search your house then the case would be dismissed even if you had something illegal in your home.
If they get a warrant then they could force the cellphone company to give them records of your text messages so you may not even know about it.
Just don't write anything illegal in your text messages and you'll have no need to worry about the police reading the text messages because they only read the text messages if someone contacts the police about a threatening or other illegal text messages was sent to their phone.