Is Speaking in Tongues Necessary?

Oneness Pentecostals (not to be mistaken as Pentecostals) teach that after repentance and faith, those who are genuinely saved are baptized in Jesus' name and then receive the Holy Spirit. Evidence of conversion is demonstrated with "speaking in tongues". The Oneness Pentecostals see speaking in tongues as proof of salvation. In a UPCI website they claim that their....

".... roots go back to an ancient Jewish feast and, more recently, to the first outpouring of the Holy Spirit on the earliest Christian believers. Because this initial outpouring of the Spirit took place on the Day of Pentecost (Acts chapter 2), those who experience a similar infilling of the Spirit are known as Pentecostals." 

In “Our Beliefs” the UPCI states: 

"The saving gospel is the good news that Jesus died for our sins, was buried, and rose again. We obey the gospel (II Thessalonians 1:8; I Peter 4:17) by repentance (death to sin), water baptism in the name of Jesus Christ (burial), and the baptism of the Holy Spirit with the initial sign of speaking in tongues as the Spirit gives the utterance (resurrection). (See I Corinthians 15:1-4; Acts 2:4, 37-39; Romans 6:3-4.)" (bracketed quotes are their own) 

There is a heavy emphasis on speaking in tongues as the "initial evidence" of baptism in the Holy Spirit. Notice that obedience to the saving gospel is the baptism of the Holy Spirit initiated by speaking in tongues. According to United Pentecostals, this is the primary evidence of being baptized in the Holy Spirit. What does the Scriptures say about this?

Romans 8:9-11 teaches that to be a Christian is to be indwelt by the Holy Spirit.

"However, you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you. But if anyone does not have the Spirit of Christ, he does not belong to Him. If Christ is in you, though the body is dead because of sin, yet the spirit is alive because of righteousness. But if the Spirit of Him who raised Jesus from the dead dwells in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit who dwells in you."
(Romans 8:9-11)

1 Corinthians 12:7-14 teaches there are different manifestations of gifts bestowed on believers that are indwelt by the Holy Spirit. The early Christians were taught...

"But to each one is given the manifestation of the Spirit for the common good" (verse 7).

The Apostle Paul did not teach speaking in tongues was the "initial evidence". Notice verse 10...

"and to another the effecting of miracles, and to another prophecy, and to another the distinguishing of spirits, to another various kinds of tongues, and to another the interpretation of tongues."

Verse 10 is only part of a list of manifestations of gifts of the Holy Spirit. If speaking in tongues is the "initial evidence" that the Holy Spirit lives in the believer, the Apostle Paul would have said, "and to EVERYONE various kinds of tongues." But he didn't. Notice he was very specific in saying "to one is given.... to another.... repeatedly (verses 7 through 10). He ends with verse 11 saying, "But one and the same Spirit works all these things, distributing to each one INDIVIDUALLY just as He wills." (emphasis mine)

Also, a manifestation of speaking in tongues is not proof that an individual has the Holy Spirit living in him/her. The Apostle Paul said, "If I speak in the tongues of men and angels, but have not love, I am only a resounding gong or a clanging cymbal" (1 Corinthians 13:1). "If I.... do not have love, I am nothing!" (verse 2).

Can a believer know if the Holy Spirit abides in them?

"In Him, you also, after listening to the message of truth, the gospel of your salvation-having also believed, you were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance, with a view to the redemption of God's own possession, to the praise of His glory." (Ephesians 1:13-14)

"By this we know that we abide in Him and He in us, because He has given us of His Spirit. We have seen and testify that the Father has sent the Son to be the Savior of the world. Whoever confesses that Jesus is the Son of God, God abides in him, and he in God." (1 John 4:13-15)

The fruit of the Spirit is "love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self–control." (Galatians 5:22–23)

Speaking in tongues is not mentioned in any of the above three epistles, yet these passages teach the believer is sealed with the Holy Spirit, Who abides in the believer and manifests the fruit of the Spirit with love, joy, peace, patience, kindness, goodness, faithfulness, gentleness and self–control.