This mystery is great (to mustērion touto mega estin). For the word “mystery” see note on Eph 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 (egō de legō). “Now I mean.” Cf. 1Co 7:29; 1Co 15:50.
In regard of Christ and of the church (eis Christon kai ̣eiš tēn ekklēsian). “With reference to Christ and the church.” That is all that eis here means.