A sign is the evidence of God's presence among His creation. Jesus tells us that it is an adulterous generation that seeks for a sign and that the only sign they would be given is the one that requires faith alone to see. It is the sign of Jonah.
"For just as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth." (Matthew 12:40)
God revealed His plan of redemption for mankind through the death and resurrection of His own son. In order to receive such a sign, it must be revealed to them.
"But when he who had set me apart before I was born, and who called me by his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles, I did not immediately consult with anyone; nor did I go up to Jerusalem to those who were apostles before me, but I went away into Arabia, and returned again to Damascus." (Galatians 1:15-17)
This is the faith that comes from God, reserved for God's elect; the substance of things hoped for, the evidence of things not seen. The children of God will ultimately walk by faith, not by sight. It is unbelief that looks for external signs as evidence of God's activities. Rather, we should wait upon the Lord, void of our own agendas, and attend to the Holy Spirit's promptings, which may very well include external signs, should He choose.
There is an abundance of God's presence that many Christians ignore on a daily basis. By not attending to the basic revelations, we are then seeking greater signs and wonders to be manifest that we may have sure evidence. God, however, will take no one further until the foundation has been first established.
Be absolutely certain of the source of any sign you embrace. Is it ours, Satan's, or God's?
Ours-"When the woman saw that the fruit of the tree was good for food and pleasing to the eye, and also desirable for gaining wisdom, she took some and ate it. She also gave some to her husband, who was with her, and he ate it." (Genesis 3:6)
Satan's-"And he said, 'See that you are not led astray. For many will come in my name, saying, ‘I am he!’ and, ‘The time is at hand!’ Do not go after them." (Luke 21:8)
God's-"For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life." (John 3:16)
"And without faith it is impossible to please him, for whoever would draw near to God must believe that he exists and that he rewards those who seek him."