1 Cor. 7:10-16 (WEB)
10 But to the married I commandānot I, but the Lordāthat the wife not leave her husband
11 (but if she departs, let her remain unmarried, or else be reconciled to her husband), and that the husband not leave his wife.
12 But to the rest Iānot the Lordāsay, if any brother has an unbelieving wife, and she is content to live with him, let him not leave her.
13 The woman who has an unbelieving husband, and he is content to live with her, let her not leave her husband.
14 For the unbelieving husband is sanctified in the wife, and the unbelieving wife is sanctified in the husband. Otherwise your children would be unclean, but now they are holy.
15 Yet if the unbeliever departs, let there be separation. The brother or the sister is not under bondage in such cases, but God has called us in peace.
16 For how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?
This is absolutely not an excuse for a Christian to divorce a spouse who leaves! It is simply saying that you don't have to try to keep an unbelieving spouse living with you, who wants to leave, but can let that person leave.