Proselytizing ( /ˈprɒsɨlɨtaɪzɨŋ/) is the act of attempting to convert people to another opinion and, particularly, another religion. The word proselytize is derived ultimately from the Greek language prefix προσ- (toward) and the verb ἔρχομαι (to come) in the form of προσήλυτος (a new comer). Historically in the Koine Greek Septuagint and New Testament, the word proselyte denoted a gentile who was considering conversion to Judaism. Though the word proselytism originally referred to Early Christianity (and earlier Gentiles), it also refers to other religions' attempts to convert people to their beliefs or even any attempt to convert people to another point of view, religious or not.

Many Christians consider it their obligation to follow what is often termed the Great Commission of Jesus, recorded in the final verses of the Gospel of Matthew: "Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen." The Acts of the Apostles and other sources contain several accounts of early Christians following this directive by engaging in individual conversations and mass sermons to spread the "good news". Evangelical Christians often use the term "witnessing" to mean discussing one's faith with another person with the intent of proselytism.