25, 26. Whom God hath set forth to be a propitiation--or "propitiatory sacrifice."
      through faith in his blood--Some of the best interpreters, observing that "faith upon" is the usual phrase in Greek, not "faith in" Christ, would place a "comma" after "faith," and understand the words as if written thus: "to be a propitiation, in His blood, through faith." But "faith in Christ" is used in Ga 3:26 and Eph 1:15; and "faith in His blood" is the natural and appropriate meaning here.
      to declare his righteousness for the remission--rather, "pretermission" or "passing by."
      of sins--"the sins."
      that are past--not the sins committed by the believer before he embraces Christ, but the sins committed under the old economy, before Christ came to "put away sin by the sacrifice of Himself."
      through the forbearance of God--God not remitting but only forbearing to punish them, or passing them by, until an adequate atonement for them should be made. In thus not imputing them, God was righteous, but He was not seen to be so; there was no "manifestation of His righteousness" in doing so under the ancient economy. But now that God can "set forth" Christ as a "propitiation for sin through faith in His blood," the righteousness of His procedure in passing by the sins of believers before, and in now remitting them, is "manifested," declared, brought fully out to the view of the whole world. (Our translators have unfortunately missed this glorious truth, taking "the sins that are past" to mean the past sins of believers--committed before faith--and rendering, by the word "remission," what means only a "passing by"; thus making it appear that "remission of sins" is "through the forbearance of God," which it certainly is not).

JFB.