To understand Iago's quote, "All seals and symbols of redeemed sin," you must examine the complete text surrounding it:
To win the Moor, were't to renounce his baptism,
All seals and symbols of redeemed sin,(330)
His soul is so enfetter'd to her love,
That she may make, unmake, do what she list,
Even as her appetite shall play the god
With his weak function.
Iago is saying that Desdemona has such a way with Othello, the Moor, until she could win his favor if he had to renounce (to give up by formal declaration) his own baptism, which is a seal and symbol of his forgiveness (redeemed sin).
Desdemona has such grace in Othello's eyes until he would renounce (deny) his own baptism, a seal and symbol of his forgiveness, if she requested it.
Of course, Desdemona would never ask Othello to renounce (deny) his own baptism. She would never ask him to deny his faith. "All seals and symbols of redeemed sin" would be referring to Othello's baptism. To be baptized is a seal or symbol of redeemed sin. Baptism, which is made publicly known during a baptism ceremony, is a seal or symbol of forgiveness. It is connected to Othello's faith in God.
Desdemona has influence with Othello at this point, but she would never ask him to renounce his own faith. She would never ask him to give up by formal declaration his baptism because his baptism is a seal and symbol of forgiveness (redeemed sin).
Iago is just making such a reference to show the hold that Desdemona has on Othello. She does have a strong hold on his heart at this point.