Your question is very appropriate and understandable. The Christian Church has called the archangel Michael “Saint Michael” or “St. Michael” for centuries.
A saint is someone who is holy in the sight of God (Romans 1:7; 1 Corinthians 1:2). Since the Bible speaks of holy angels (Mark 8:38; Jude 14; Revelation 14:10), “saint” is a word that can be attributed to them. (Not all the angels remained holy – 2 Peter 2:4.)
The minor festival of St. Michael and All Angels provides Christians with an opportunity to praise God for using his holy angels to protect them (Hebrews 1:14).