1 Thessalonians 1

As Paul begins his letter to the Thessalonians, he reminds them of their conversions writing -

"For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance..."  (New Testament | 1 Thessalonians 1:5)

I believe that is significant.  Obviously, the gospel is preached in word, both written and verbally.  But Paul also testifies that the truth of gospel of Jesus Christ is also testified by the power of the Holy Ghost which also provides assurance.  Paul then later writes how he and his fellow missionaries brought them the word of the gospel -

For our exhortation was not of deceit, nor of uncleanness, nor in guile:

"But as we were allowed of God to be put in trust with the gospel, even so we speak; not as pleasing men, but God, which trieth our hearts.
5 For neither at any time used we flattering words, as ye know, nor a cloke of covetousness; God is witness:
6 Nor of men sought we glory, neither of you, nor yet of others, when we might have been burdensome, as the apostles of Christ.
7 But we were gentle among you, even as a nurse cherisheth her children..."  (New Testament | 1 Thessalonians 2:3 - 7)

For -

"Ye are witnesses, and God also, how holily and justly and unblameably we behaved ourselves among you that believe:
11 As ye know how we exhorted and comforted and charged every one of you, as a father doth his children,
12 That ye would walk worthy of God, who hath called you unto his kingdom and glory."  (New Testament | 1 Thessalonians 2:10 - 12)

This is how missionaries are to teach the gospel throughout the world -

"...because, when ye received the word of God which ye heard of us, ye 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."  (New Testament | 1 Thessalonians 2:13)