Leon Morris wrote the Pillar commentary on Romans. I decided to look up what he says about Romans 3:25, which he translates, God presented him as a sacrifice of atonement ...
Morris writes, 'There are problems with the ... expression, which the KJV rendered, "a propitiation" ... Part of the trouble is that "propitiation" is neither well-known nor a well-used word today, and translators like to employ something simpler. But the major reason is that propitiation means the removal of God's wrath and ... some commentators find the concept of divine wrath distasteful and unworthy; so they write it out of Scripture ... There are two major reasons for rejecting this approach. One is the meaning of the word Paul uses ... detailed examinations [in a footnote he names a number] ... show that the word means "the removal of wrath". The other is context. Paul has mounted heavy artillery in sections 1:18-3:20 to show that all are sinners and are subject to the wrath of God. But unless the present term means the removal of wrath he has left them there, still under God's wrath.'
He later says that 'it is most important that what God did in Christ averted the divine wrath from sinners.'
Morris disagrees with the idea that the word should be understood as "mercy seat" (see comments by Moo below). In a footnote he mentions Denny who says that "A 'mercy-seat' is not such a self-evident, self-interpreting idea, that the Apostle could lay it at the heart of his gospel without a word of explanation.' Although Moo claims that 'Paul's letters furnish abundant proof that he expected his Gentile readers to be fully conversant with the OT.'