What is the biblical basis for defining 'atonement’ as 'at-one-ment with Christ'?

I’m looking for answers from Protestant Trinitarians as this seems to be a fairly recent explanation, currently in vogue in some groups (mainly evangelical, I would suppose). The basis for my query is the following scripture texts (all A.V.): > “And not only so, but we also joy in God through our Lord Jesus > Christ, by whom we have now received the atonement.” Romans 5:11 > > “And Aaron shall make an atonement upon the horns of it once in a year > with the blood of the sin offering of atonements: once in the year > shall he make atonement upon it throughout your generations : it is > most holy unto the Lord. …when they give an offering unto the Lord, to > make an atonement for your souls, and thou shalt take the atonement > money of the children of Israel, and shalt appoint if for the service > of the tabernacle of the congregation; that it may be a memorial unto > the children of Israel before the Lord, to make an atonement for your > souls.” Exodus 30:10 & 15-16 > > “For the life of the flesh is in the blood : and I have given it to > you upon the altar to make an atonement for your soul : for it is the > blood that maketh an atonement for the soul.” Leviticus 17:11 I note that modern translations use the word ‘atonement’ more frequently. However, I am not looking for simple agreement with what those texts say, or any subjective opinion as to what atonement means to individual Christians, but of ***what this one word, ‘atonement’ actually means, in context, and whether it could be misleading to say it means ‘at-one-ment’ (not least because that phrase of cobbled-together words explains nothing, in and of itself.)***