1 Chronicles 12:18
Then the spirit came upon Amasai, who was chief of the captains, and he said, Thine are we, David, and on thy side, thou son of Jesse: peace, peace be unto thee, and peace be to thine helpers; for thy God helpeth thee. Then David received them, and made them captains of the band.
The chapter supplies a fuller parallel account of the tribes assembling at Hebron to acknowledge David as king, recording the events that led to his rule over all Israel. About 350,000 fighting men were sent for that assembly, which became a three-day festival. The contingents from each tribe who joined his army were mighty, swift, skilled, wise, and wholehearted men, and their loyalty is emphasized. His supporters rallied to him as king because they recognized that God was helping him.
The Spirit of Jehovah came upon Amasai, clothing or inspiring him to speak, as an example of God's Spirit entering a person to serve as His mouthpiece. Amasai and his men declared that they belonged to David and were on his side, wishing peace to him and to his helpers because God helped him.
That declaration of being on David's side means wholehearted allegiance to Jesus. It means taking Him as Savior and Sovereign, giving oneself fully to His service, and refusing compromise with the world and sin.