For I am sure that neither death nor life, nor angels nor rulers, nor things present nor things to come, nor powers, nor height nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord.
For if, after they have escaped the defilements of the world through the knowledge of our Lord and Savior Jesus Christ, they are again entangled in them and overcome, the last state has become worse for them than the first. For it would have been better for them never to have known the way of righteousness than after knowing it to turn back from the holy commandment delivered to them.
2 Peter 2:20-21
The Claim: Romans says that it's impossible to fall from grace, but 2 Peter talks about it being possible.
The Explanation: Both passages are talking about 2 entirely different subjects. Romans is talking about God's love, and more specifically, being separated from it. What Romans describes is that nothing can force you to be separate from God's love, which is true because God's love is unconditional. Whether you have received God's forgiveness or not does not change God's love for you, because as Romans states, nothing can force a separation between you and God's love.
The passage in 2 Peter is discussing those who have the knowledge of the Lord Jesus Christ and have chosen to turn their back on what Christ did for them on the cross. This passage is describing people who would appear to be Christians, but later choose to reject God. This is not to say that God stops loving those people, because as Romans says, nothing can separate us from the love of God. In order to complete this idea, we need to take a look at the next verse.
What the true proverb says has happened to them: "The dog returns to its own vomit, and the sow, after washing herself, returns to wallow in the mire."
2 Peter 2:22
Now with this verse, we can clearly decipher that 2 Peter 2:20-22 depicts those who appeared to be Christians, but have turned away from God and back to their old sinful ways (as a dog returns to its own vomit). The proverb Peter is quoting here is Proverbs 26:11. But this proverb does not say that God stops loving these people, and therefore there is no contradiction here.