It seems that you may not be the only one with this question. As you have identified, the phrase "only in the Lord" is teaching that a widow can marry anyone she wishes that is a follower of Christ.
This phrase may have required clarification however as it is addressed in the follow-up epistle to the church in Corinth in 2 Corinthians 6:14 which makes it clear that this requirement extends to all believers:
Do not be yoked together with unbelievers. For what do righteousness and wickedness have in common? Or what fellowship can light have with darkness?
This is also implied earlier on in Chapter 7 in verses 12-16
To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy. But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called you to peace. For how do you know, wife, whether you will save your husband? Or how do you know, husband, whether you will save your wife?
Clearly what the author is saying is that if you become a follower of Christ while married to someone who is not a follower of Christ, while this may not be ideal, you should remain married. If your spouse divorces you or dies however, any future marriages should be with a follower of Christ, should you marry at all according to Paul.