testify unto, (also) bear witness (with)

Boyd’s Bible Dictionary:

(see). Under the Mosaic law at least two witnesses were required to establish a capital charge (Num. 35:3030Whoso killeth any person, the murderer shall be put to death by the mouth of witnesses: but one witness shall not testify against any person to cause him to die. (Numbers 35:30); Deut. 17:6-76At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death. 7The hands of the witnesses shall be first upon him to put him to death, and afterward the hands of all the people. So thou shalt put the evil away from among you. (Deuteronomy 17:6‑7)). False swearing forbidden (Ex. 20:1616Thou shalt not bear false witness against thy neighbor. (Exodus 20:16); Lev. 6:1-71And the Lord spake unto Moses, saying, 2If a soul sin, and commit a trespass against the Lord, and lie unto his neighbor in that which was delivered him to keep, or in fellowship, or in a thing taken away by violence, or hath deceived his neighbor; 3Or have found that which was lost, and lieth concerning it, and sweareth falsely; in any of all these that a man doeth, sinning therein: 4Then it shall be, because he hath sinned, and is guilty, that he shall restore that which he took violently away, or the thing which he hath deceitfully gotten, or that which was delivered him to keep, or the lost thing which he found, 5Or all that about which he hath sworn falsely; he shall even restore it in the principal, and shall add the fifth part more thereto, and give it unto him to whom it appertaineth, in the day of his trespass offering. 6And he shall bring his trespass offering unto the Lord, a ram without blemish out of the flock, with thy estimation, for a trespass offering, unto the priest: 7And the priest shall make an atonement for him before the Lord: and it shall be forgiven him for any thing of all that he hath done in trespassing therein. (Leviticus 6:1‑7)).

Concise Bible Dictionary:

The testimony or evidence adduced or given in confirmation of an assertion, and so often used judicially. The term also sometimes speaks simply of an expression of mind or feeling. Until God intervenes in power to establish His own purpose in regard to this world, He maintains a testimony to that which He will assuredly accomplish.
The words μαρτυρέω, μαρτυρια, and, μαρτύριον are translated both “testimony” and “witness.” The idea runs all through the scriptures in respect both to God Himself and to His people. Paul declared before the heathen at Lystra that God “had not left himself without witness” as to His existence and His goodness, in giving rain and fruitful seasons, filling their “hearts with food and gladness” (Acts 14:1717Nevertheless he left not himself without witness, in that he did good, and gave us rain from heaven, and fruitful seasons, filling our hearts with food and gladness. (Acts 14:17)). The invisible things of God are testified of, “being understood by the things that are made, even His eternal power and Godhead, or divinity” (Rom. 1:19-2019Because that which may be known of God is manifest in them; for God hath showed it unto them. 20For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse: (Romans 1:19‑20)).
God having for fifteen hundred years manifested His patience towards the guilty antediluvian world, He, after warning the people by the preaching of Noah, bore witness to His righteousness and His power by the deluge, and at the same time manifested, His grace in saving Noah and his family in the ark.
The witness which God vouchsafed of Himself to Abraham was that He was “THE ALMIGHTY GOD”; to Moses it was “I AM THAT I AM”; and to Israel, “JEHOVAH.” The ark was often called the “Ark of the testimony,” and the tabernacle was the “Tent of witness,” the witness of good things to come. To Nebuchadnezzar God was witnessed to as the “GOD OF HEAVEN.” To the Christian He is “GOD AND FATHER.”
Israel were of old God’s witnesses, and will also be in the future.
Peter and John were witnesses of the truth before the council, so that they took knowledge of them that they had been with Jesus (Acts 4:1313Now when they saw the boldness of Peter and John, and perceived that they were unlearned and ignorant men, they marvelled; and they took knowledge of them, that they had been with Jesus. (Acts 4:13)). Stephen also was a true witness, and his testimony led to his becoming a martyr (μάρτυς). In Hebrews 11 is given a “great cloud” of witnesses to the principle of faith in Old Testament saints, some of whom were also martyrs. God will to the last have a testimony on earth as seen in His “two witnesses” of Revelation 11.
The Church, in the absence of the Lord Jesus, is the vessel of the testimony of Christ, hence Christians should be in their whole life and deportment true witnesses to the rejected Christ. The testimony of the church is characterized by—separation from the world; devotedness to the interests of the Lord Jesus on earth; faithfulness to the truth; unblamable moral conduct; and indeed, as the pillar and ground of the truth, by everything that becometh godliness.

Strong’s Dictionary of Greek Words:

Number:
4828 (find all occurrences in KJV Bible)
Greek:
συμμαρτυρέω
Transliteration:
summartureo
Phonic:
soom-mar-too-reh’-o
Meaning:
from 4862 and 3140; to testify jointly, i.e. corroborate by (concurrent) evidence
KJV Usage:
testify unto, (also) bear witness (with)