This mystery is great (το μυστηριον τουτο μεγα εστιν). For the word "mystery" see Ephesians 1:9. Clearly Paul means to say that the comparison of marriage to the union of Christ and the church is the mystery. He makes that plain by the next words.

But I speak (εγω δε λεγω). "Now I mean." Cf. 1. Corinthians 7:29; 1. Corinthians 15:50.

In regard of Christ and of the church (εις Χριστον κα [εισ] την εκκλησιαν). "With reference to Christ and the church." That is all that εις here means.