No, I said the could not have been spiritually placed in Christ before Christ died on the cross. Salvation occurs when God credits our faith in Christ as righteousness and places us spiritually in Christ where we undergo the circumcision of Christ and arise in Christ a new creation. Then we are "sealed" in Christ with the Holy Spirit. So by the numbers, 1, Christ died, 2, we believe, 3, God, who knows our heart credits our "all in" faith as righteousness, and 4, God spiritually places us in Christ and seals us with the Holy Spirit forever. We are now saved. So belief provides our access to the grace in which we stand, Romans 5:2, but does not save us. God does, for He has mercy on whom He has mercy.
So yes, they could have believed from their heart Jesus was the Messiah before He died on the cross. We can speculate that perhaps they did not until the saw the risen Christ, like Peter and Thomas, but to say none believed before the cross does not have specific support in scripture. However, they could not have been spiritually placed in Christ until after Christ died, becoming the propitiation for sin. So the sequence may not be fixed for some of the disciples, but it is a moot point because now the sequence is fixed, none of us believed before Christ died and ascended.
One more point, it is God who credits our faith, as flawed or grand as it may be, as righteousness. So I could hold to all sorts of fouled up doctrine, but believe in my heart Jesus died for me, paid for my sins with His blood because He loved me, and therefore I love Him. Who is to say, God could not credit that simple faith of a child of 6 or 7 and spiritually place the child in Christ. Certainly not me.