If assent and trust were synonyms, then either both would mean cognitive conviction or else volitional reliance. Conviction of truth (assent) could never give way to reliance upon truth (trust). If assent and trust are indistinguishable concepts and, therefore, mean the same thing, then it would be unintelligible to say that we rely upon anything we believe; nor would it be sensible to think that we believe anything we rely upon. Intellectual assent without reliance leaves no room for trusting in Christ.
It has been argued by some Arminians (usually antinomians) and Calvinists (usually Clarkians) that we are justified by belief alone and that receiving and resting in Christ unpacks what it is to believe. In other words, receiving and resting in Christ is considered a figure of speech by which belief in Christ can be defined. It’s alleged that trusting in Christ alone does not complete justifying belief because trust is synonymous with belief. Therefore, to add receiving and resting in Christ to belief is either redundant or to add something additional to the instrumental cause of justification. The first deviation from the aberrant assent-alone view would be considered by those who hold to it a matter of muddled thinking, but the gospel would remain intact although jumbled. Whereas the second construct would undermine the grace by which we are saved, appropriated by belief alone.
Those who promote the belief alone view are sometimes met with tedious rejoinders such as the false dichotomy “we’re saved by Christ not propositional belief.” Notwithstanding, more serious objections have been raised against the belief alone position aimed at the group’s insistence upon reducing justifying faith to mere assent. This is where things get a bit nuanced.
Not All Beliefs Involve the Will
Most of the things we assent to, whether a priori or a posteriori, are not volitional. One does not will to believe that God exists any more than one wills to believe the rose is red. These are mental assents that are not discursive; they are immediate and without reflection. The will is bypassed.1 However, the gospel always engages the will as the unbeliever counts the cost and by grace abandons all hope in himself while looking to Christ alone, finding rest in Him. Accordingly, it is inadequate to reduce justifying faith to belief alone when belief is reduced to intellectual assent without remainder.
Equivocal Language Confuses
It is at this point some assert that assent is synonymous with trust in Christ. In this context it is opined that to assent to Christ dying on the cross for my sins is to trust the proposition is true. Albeit the premise is true, this observation turns on a subtle equivocation over the word trust. Indeed, to trust a proposition is true is no different than to assent to its truth. So, in that sense trust and assent are synonyms. However, to trust that something is true is not the same thing as to trust in that something. The latter idea of trust carries the meaning of reliance, whereas the former use of trust merely conveys an intellectual assent that might or might not be accompanied by the reliance sort of trust. Accordingly, to argue that trust and assent are synonymous is to deny the need to willfully trust in Christ alone for salvation.