God is all-just because He is completely honest and fair with everybody. He promises to reward us for our good deeds and to punish us for our evil deeds. Yet, He is always merciful to those who turn to Him. (See Q. 14.)
Yes, God alone, the Creator of all things, is all-wise. He knows what is best for all of His creatures.
God is all-holy because He is infinitely full of all that is good and lovable.
God is all-merciful because there is no end to His mercy. As often as we are sorry for our sins and confess them, so often will God forgive us.