Not in Word Only

The Apostle then having just listed 3 things that he saw, listed two things that he knew.

He Knew that they were Loved of God and He Knew that they were the Elect of God

In other words, The evidence of the Three Essential Graces (Faith, Love and Hope), coupled with signs of their Source and Sincerity in those Graces, convinced the Apostle Paul that they were a People upon whom God had Set His Electing Love

The Loving Election of Individuals unto Salvation, is something that is Secret and known only by God Himself
However, it is also something that manifests itself in men, by the it Effects that flow from it

Having said that the Effects are one way of knowing, the Apostle now in verse 5, shows us that we can also know by the Effectiveness of the Gospel when it first came unto them
What Theologians call Efficacy = capacity of the Gospel to produce the desired result or effect;

The Gospel was Effective, or successful in bringing about the desired results

It speaks of the strength or the sufficiency of the Gospel to do the work of God in the lives of men
This is the way Paul spoke about the Power of the Gospel in 2:13

“For this cause thank we God without ceasing because when ye received the Word of God which ye heard of us, you received it not as the word of men, but as it is in truth, the Word of God which effectually worketh also in you that believe.”