ὁ λαβὼν αὐτοῦ τὴν μαρτυρίαν ἐσφράγισεν ὅτι ὁ Θεὸς ἀληθής ἐστιν.

1. ὁ
   the
2. λαβὼν
   having taken
3. αὐτοῦ
   his
4. τὴν
   the
5. μαρτυρίαν
   testimony
6. ἐσφράγισεν
   has sealed
   he sealed, sealed
7. ὅτι
   that
   because
8. ὁ
   the
9. Θεὸς
   God
10. ἀληθής
    true
11. ἐστιν
    is
12.  
    .
     

The one who has received his testimony has certified that God is true.