Jamieson, Fausset and Brown on Titus 1:4

4. Titus, mine own son--Greek, "my genuine child" (1Ti 1:2), that is, converted by my instrumentality (1Co 4:17; Phm 10).

after the common faith--a genuine son in respect to (in virtue of) the faith common to all the people of God, comprising in a common brotherhood Gentiles as well as Jews, therefore embracing Titus a Gentile (2Pe 1:1; Jude 3).

Grace, mercy, and peace--"mercy" is omitted in some of the oldest manuscripts. But one of the best and oldest manuscripts supports it (compare Notes, see on 1Ti 1:2; 2Ti 1:2). There are many similarities of phrase in the Pastoral Epistles.

the Lord Jesus Christ--The oldest manuscripts read only "Christ Jesus."

our Saviour--found thus added to "Christ" only in Paul's Pastoral Epistles, and in 2Pe 1:1, 11; 2:20; 3:18.