He promised afore (προεπηγγειλατο). First aorist middle of προεπαγγελλω for which verb see on 2. Corinthians 9:5.

By (δια). Through, by means of, intermediate agency like Matthew 1:22 which see.

In the holy scriptures (εν γραφαις αγιαις). No article, yet definite. Perhaps the earliest use of the phrase (Sanday and Headlam). Paul definitely finds God's gospel in the Holy Scriptures.