Can the phrase "Since we believe in the existence of God, then nothing is impossible" be interpreted as one of the sayings of deviant sects, and does this interpretation make it a cause for perdition and torment?

The phrase mentioned is not absolutely correct, because belief in Allah is not limited to merely believing in His existence. Furthermore, there are matters that Allah has made impossible for humans, such as producing a text similar to the Holy Quran. Therefore, one should avoid phrases that might imply wrong meanings. However, if you did not intend a wrong meaning, and the phrase could imply a correct meaning, such as an exaggeration of trust in Allah, then there is no sin upon you. It is recommended to change the signature to a phrase that does not suggest a wrong meaning.