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 &amp; 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.)
I first came across the idea of “at-one-ment” with Christ after I become a Christian (as a mature adult).  It seemed a simple explanation but later I came to see it more as a ‘dumbed-down sound-bite’.  It conveyed the notion that, because Christ had done something, people could then become ‘one with Christ’.  Oh, but what danger lurked behind that view!  Christians are people who have died WITH Christ, for if that never happens, they can never be at one with him. 2 Timothy 2:11 says:

For if we be dead with him, we shall also live with him. If we suffer, we shall also reign with him.

According to the ‘at-one-ment’ pseudo-explanation of the word ‘atonement’ the impression is given that because Christ has done something, we are sorted.  Jesus has done it all.  Well, so he has – he has achieved a finished work via the cross and we can do nothing to add to that completed salvation. But the danger lies in then thinking we can just verbally agree with the wonder of what Christ has done for us, thank God for it, and that’s us united with Christ.  Is it really such a passive matter for believers?  Is this not the sort of shallow thinking behind the notorious “Sinner’s Prayer”?
Over the five years I spent in a U.K. Baptist church I learned much more, and the depth of meaning behind the enormity of what is involved in reconciling sinners to God began to dawn.  That, however, did not nullify the fact that I had become a Christian at the outset.  It just took a while for me to appreciate the significance of what being crucified prior to being raised with Christ, meant. But the way some believers used dumbed-down sound-bite explanations really hindered my grasp of matters.
It took me a while to understand the significance of Romans 6 but a key point came from verses 8 to 12:

Now if we be dead with Christ, we believe that we shall also live with him…  Likewise reckon ye also yourselves to be dead indeed unto sin, but alive unto God through Jesus Christ our Lord.  Let no sin therefore reign in your moral body. (NIV)

Jesus explained what is required of all Christians in Matthew 16:24-25:

If any man will come after me, let him deny himself, and take up his cross, and follow me. For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it.

That was my ‘lightbulb moment’.  Verbally agreeing with the wonder of what Jesus did by dying on the cross was not my dying to sin.  That was not me disowning myself, picking up my cross daily, and losing my life for Christ’s sake in the process.  It had to get personal, not just be something done in the past by Christ which, two thousand years later, moved me intellectually or emotionally into a statement of verbal agreement.
My answer is that claiming biblical atonement means “at-one-ment with Christ” explains nothing and may even prevent sincere people grasping the enormity of what it means to enter into the death of Christ, which has to happen before we can be raised with Christ.