This is the way it is with signs. The illustration is a road sign warning drivers that there may be children playing in the area. As it is, on a pole beside the road, the sign is not a 'mere sign', but conveys information about a possible danger. Take the sign, old and attractive as it is, and put it in a transport museum, and you do have a mere sign - it has been taken out of its proper context and no longer signifies anything. Take an example where the sign and the thing signified are always together, the speed limit. If a man is driving along a road and sees a round sign with a red border and the number 30 in the middle, he may say "it is just a sign", but if he drives fastyer that 30mph, he has broken the law, and if there's a speed camera in the area, he is going to find out that was no mere sign! So a sign, if it is actually in use, is never a mere sign, but a sign of a present reality.
So it is with the sacraments. In the Lord's Supper God is speaking to us, and conveying Christ's blessings to believing hearts. Therefore we sing with Mr. Spurgeon, "Amidst us our Beloved stands." The elements are not memorials of an absent Christ, but signs of a present Christ, known by faith. In baptism, God speaks to the one baptised assuring that person of his interest in Christ - again, to faith. Faith receives the blessings of the sacraments, and unbelief does not.