Acts 14:3

‘For a long time therefore they tarried there speaking boldly in the Lord, who bore witness to the word of his grace, granting signs and wonders to be done by their hands.'

Meanwhile Paul and his party were able to continue on unafraid, encouraging the persecuted believers (‘the brethren') and no doubt also themselves seeking to build up popular support. Thus they were able to remain there a long time, and continue to speak boldly in the Lord, bearing witness to ‘His grace', that is, proclaiming the Good News of the unmerited favour that God had revealed towards man and what through His unmerited favour they could receive in Jesus Christ. At the same time the Lord backed them up by granting signs and wonders to be done by their hands. These linked them with the ministries both of Jesus and of the Apostles Acts 2:22; Acts 2:43; Acts 4:30; Acts 5:12; Acts 6:8; Acts 7:36 compare Galatians 3:4). The work of the same Holy Spirit was clearly going forward among the Gentiles.

‘In the Lord' almost certainly refers to the Lord Jesus Christ, although the ambiguity is probably intentional. Jesus is ‘the Lord' in every sense, and the word is the word of His grace (compare Acts 13:43 where it was ‘the grace of God').