In Act 1, Othello believes Iago is honest and trustworthy.
At the end of Act 1, when he sets off to fight the Turks, he entrusts his new wife to Iago’s care:
‘Honest Iago / My Desdemona must I leave to thee’ (1:3).
Iago makes it very clear to the audience, however that he will use Othello’s trust against him:
‘He holds me well / The better shall my purpose work on him’ (1:3)
In Act 2, Othello trusts that Iago is loyal both to him and to Cassio and has no idea Iago caused the fight Cassio gets into with Montano.
‘I know Iago / Thy honesty and love doth mince this matter, / Making it light to Cassio.’ (2:3)
Othello places so much trust in Iago’s honesty and loyalty that he believes Iago’s suggestions that Cassio and Desdemona are having an affair rather than trust in the loyalty of his wife and his lieutenant.
‘This fellow’s of exceeding honesty’ (3:3)
Othello trusts Iago with his plans to kill Desdemona and also trusts Iago to kill Cassio for him. When he hears Cassio’s cries for help, Othello says:
‘O brave Iago, honest and just, / Thou hast such noble sense of thy friend’s wrong’ (5:1)
Othello is shocked that Iago betrayed his trust so badly and calls him a:
‘demi-devil’ that ‘hath thus ensnared my soul and body’ (5:2)